I have 3dc (3 years apart each) and all their family positions have advantages and disadvantages for them:
Dd1: had 3 years on her own, but also we tended to continue doing things she enjoyed and not things she didn't. So she got her choice in that. Also after school activities, what limited her was money and her time, whereas the other two I have to consider what we can fit in round what we already do. When she's done things it's new, and is often seen as more important (eg homework)
But she also had to be responsible early. I had horrendous morning sickness with dd2, and by the end of pregnancy she was both capable of buying herself lunch (watched by me as the shop has a large glass window) and putting jacket potatoes in the oven (oven off, turning it on after they were on, leaving for daddy to get home to do it). At 6yo she had 2 siblings so had to take responsibility for herself.
She also has our "experiments" that went wrong. For example, we realised when she was year 3 that the amount of pocketmoney we gave was about 1/4 what others had, and she was expected to use it for more. So we altered it to look more comparible-but she'd then had over 3 years of the lower rate.
Dd2: She had her trail blazed by dd1-so preschool, school, some of the activities dd1 had done it first and so we were better prepared and didn't make the mistakes we did with dd1. She was often offered activities dd1 did early-ie younger than the official age, because she was seen by the leaders as around and keen to do it, so they would offer it to her. She's got a lot of things simply by being there when dd1 did it that she wouldn't have. She had the fun of having a friend to play with from the youngest age.
But she has all the second hand stuff, she had a younger sibling at 3yo so had to learn to do some things by herself. She's also found that whatever she does/achieves has often been done by dd1. She doesn't get the choice that dd1 had for activities, and has tended to follow dd1 (not in everything) rather than having an open choice. She also always has had a sibling joining in (eg cooking a cake)
Ds: He's 6yo and things that dd1 had to do for herself, I can find myself doing things like putting his shoes on that there's no way I'd have done for dd1 or dd2. He gets access to things we wouldn't have considered until older because we're there. He has two older sisters (oldest can be very devoted) who will do things like get drinks, make his bed, mend things, find things etc. I also learnt to drive just before he was born so he had access to things the girls didn't, plus he had 3 years of both girls at school with me on my own, so he did things especially for him then.
But as he said once "every time I sit down I have to fetch one of my sisters!" and it can be like that at times. Again, he has the one of my sisters has done it first feeling. If he says XXX is happening at school, he'll often have one or both sisters telling him exactly what happens (even when wrong). I have to be careful not to say to him "no you're too young" only to remember he's 2 years older than dd1 and dd2 were when they did it-he is young for his age, but he is the smallest, when they seemed old and responsible at his age. He also ends up with things left-he's still in the toddler bed that they moved out of at 3yo because the next one needed it and things like that.