I suspect it's less about age etc, as obviously any child changes their age and most parenst don't care that much about gender. I think it's more about how you imagine the child will be throughout their childhood and into teenaged years and youg adulthood.
For example, people sometimes post on here saying stuff like "we are a wonderful loving family, we are so busy and active, we do sport, hike, play the guitar together, camp, -wouldn't we be the perfect family to adopt, we have so much to offer a child? " .
thing is, is, it's fairly unlikely that an adopted child will come from a birth family who do all this , nor are they likely to have done it in foster care. So there's a pretty good chance they will not enjoy doing all the stuff your wonderful family now does. And even if they belive they might like it, they might not be able to stop themselves sabotaging it. Camping might trigger frightening memories for them. They probably haven't learned the perseverance required to learn musical instrument. Their social worker might not let you take them camping anyway as its not been risk assessed.etc etc
And yes of course, I know that any birth child might not enjoy these things. But they are much much more likely to, because they have the genetic and personality similarity and they've been doing it from an early age. Plus they are much more likely just to go along with things than a child who has lived elsewhere and experinced other lifestyles
I know you have a child already and IIRC, one with a stong personality. What will you do if a new child doesn't want to join in the activities that you do together now? If they want a totally diffenet kind of diet? Want to watch different tv? Have different values and ambitions?
Many adoptive families have educational qualifications and good jobs. They want that for their adopted children. Odds are that the child comes from a background without these things. They may have learning difficulties. They might see no value in education. How will that be for you? How will you feel if your other child goes to university and your youger one leaves school at 16 and goes onto the dole? If your neighbours, friends and family belive you are treating them differently because they are "not your own".
What if a child rejects your political , religious or moral frame work?( This is a question often asked of paremst who practice a religion, but my experince is that in fact some of the most militant believers are agnostics or atheists, who insist that their children must not be taught anything of other belief systems ) .
Will you miss church every other week because the child doesn't want to go?
Teenaged years -mental health problems and addictions have a genetic link. Your adopted teen is more vulnerable to these things. You will get less help than bio parenst in the same situation. How will you cope?
Etc etc. etc
sorry if this sounds so negative but I'm afraid an exercise about loss will inevitably bring up difficult feelings