I agree that age is a bit more tricky because you can end up having a child with a different chronological age to their developmental stage, but it is also worth considering other factors. The usual ones of considering the child's background etc, what you can manage, but there's also some other things you might want to consider. None of these are particularly make or break, but worth a thought.
It might depend on what support you have available. For example, do you have someone who is physically able to support with a baby or toddler (e g lifting and carrying) in your network for respite? Is anyone experienced in dealing with a teething two year old? Anyone who could advise on primary senco relationships?
Do you have friends with children similar ages? Would that be better or worse? (Comparison is the thief of joy and all that). We found that relationships with my extended family are trickier because our two interrupted the cousin birth order, and being close on age to what was once the only grandchild has meant they don't always gel well together as my nephew felt threatened and my two can be feral at a family easter egg hunt. I can imagine of you have lots of family or friends with children, you want them to be friends, but it might be tricky to integrate them if they are the same age, whereas if there is a significant age gap its easier for everyone to look kindly on "the baby" of the group. Alternatively,similar ages can provide fellowship when you are all in the trenches together, and it's not like you are 3 or 4 years "out".
For example, I found it personally socially harder to integrate into my older sons nursery circle because they had been going since September and had already cliqued up. He was also too old for most of the stay and plays and rhyme times, which meant i was effectively on my own with a 3.5 year old trying to entertain him. Museum stuff started at 5 years old, so it was a funny old age. He had also been to nursery prior to being with us, so we got to observe him there and that has been invaluable. However, he did have another thing to grieve the loss of, and it was a major factor in us putting him in nursery earlier in placement than is typical, because he was used to and needed that level of stimulation.
My younger one started nursery at the appropriate time, and got the most out of the local stay and plays because they were for his age range, so I made some more friends/contacts through that.
However, childcare costs are an absolute killer, and can seriously impact your earning potential. An older child will go into school and you are paying mainly for wraparound care.
Its also worth thinking about what ages are you interested in. I don't mind babies, but I'm not all gooey eyed for them, and my siblings and cousins provide plenty of nieces and nephews where I can get baby snuggles if I want. My two came walking and just about talking. I really enjoyed looking stuff up I thought they would like and felt it was a big achievement when they did indeed like it, and I could basically use my adoption leave to take them on days out and do cute stuff with them (between the meltdowns!)
However, longer term its worth considering something that our therapist told us- if it happened to them pre verbally, it needs to be processed non verbally. An older child may have been exposed to more stuff they need to process, but may have already started that in FC. A younger child is less able to communicate their needs so it can feel you are sometimes less able to meet them.i know when our two year old came, he was on the cusp of speaking, but he could understand loads of stuff. His frustration was palpable that he didn't have the language to express or explore any of the things he was experiencing. Our 3 year old could be told stuff and retain it because he could use language to organise his experiences and retell his narrative. It's going to be traumatic to move to your care, but their capacity to understand and process it makes a big difference. The youngest that we adopted at 2 has few of his own conscious memories of FC (obviously the body keeps the score etc) so he is dependent on us providing that narrative framework for him. The eldest we adopted at 3 does have conscious memories of his own, and has his own narrative that we are still unpicking and clarifying for him. He has a lot of wishful thinking, and we have found it helpful to differentiate between babyhood, toddlerhood and childhood.
Also, with an older child, you might need more consent- you won't be able to count on doing stuff like changing their nappy or hand feeding them to build the level of parental dependence, so you might need to consider how to be intentional about building that in other ways. It can possibly mean a more equal relationship between parent and child, but a still rewarding one nevertheless. It might mean in some ways parenting a bit more outside of typical ways, even for adoption, though. Older children have more agency, so can both be reasoned with but also are less easy to control- a 2 year old kicking off in a super market is relatively typical, but you can pick them up and show them something shiny to diatract them. A 5 year old can be still picked up and moved on/distracted but it is harder and they definitely remember the next time you are in Asda!
You might want to think about what support networks are available professionally- we have an excellent children's centre near us but they only go up to 5. They are expanding family hubs furrher afield that aim to provide a similar service to up to 18, but I don't know if it will provide the same level of wraparound support to the same level as it is new. A 6 year old might miss out on this, or even a 4 year old, as provision takes time to put in place.
As many others have said, an older child might have more challenges be known, and have their personality formed a little more. Depending on what you think of as older really, but we only started to see our youngest's diva personality truly show just before he turned 3. It's been a blessing to watch it unfold, when we wouldnt have necessarily have been had i been back at work, but also to have been able to be present to shape some of it was great.
You do miss put on some stuff, but there are also plenty of firsts. You might not see their first steps, but you will see their first steps towards you when they need you. You might not hear their first words, but you will hear the first time they call you mum.
Parenting makes you greedy for every part of your child's life, and if you adopt an older child then there may well be a part of you that forever mourns and grieves those first years because you just want and love them so hard that you wish you had every moment. But then again, it is those years that mean your child is who they are, and are such an intrinsic part of them that it is mixed up in a true acceptance and love for who they truly are.
No matter what age the child is, you are essentially going to start living with a stranger, and they are going to be living with a stranger too. Their one and only job is to try and survive, whereas yours is to try and survive yourself, but also meet as many of their needs as you possibly can whilst also trying to enjoy the process a bit. The age and abilities of the child or children is a factor in this, because yes you will grow with them- the parent of a single newborn baby is not necessarily the same as the parent of a single 6 year old. You might morph from one to the other, but it is OK to say that, given the choice, you might actually be better suited to jumping into parenting a 4, 5 or 6 year old because you can do more stuff with them, reason with them, are happy to do things differently, or just that you vibed with their personalities at a stay and play and think they are the special kind of little person you could spend lots of time with and parent well. It's also OK to think that actually it's important to you that you get to have as many of those experiences in your child's early life because you are greedy for every moment and want to drink it all up.
We found going to stay and plays really helped- there were some kids on paper who were perfect for us, and when we got there we both just didn't click with them. There were definitely others who on paper didn't meet what we thought we wanted but we completely vibed with, and I could have imagined bringing them home.
As you say, it is complicated and personal, but also intermingled with lots of other factors that interrelate. The above are just some ideas to consider, and may or may not be relevant to your situation, so feel free to disregard what doesn't apply!