May, there are a few aspects, totally just my opinion:
- Not wanting to disturb the 'natural' hierarchy within the family
Your first-born is your first-born and should not be usurped from that position. Which is why it is nearly unheard of that anyone would adopt a child who is older than, or indeed the same age as, any children they have already. Younger siblings joining the family is seen as more 'natural'. (I don't personally totally agree with this. Referring to 'the natural order of things' tends to annoy me, it is frequently used where no other arguments seem to be found... However, the 'naturalness' argument is something you won't get around. Consider it as a given, unless you move to the US or such, that if you adopt, the adopted child will be younger than your BC.)
- developmental age not the same as chronological age
Many children who need adopting have had such experiences and backgrounds which mean that their developmental stage may significantly lag behind their chronological age. This means that even if you were to adopt a child of similar age to your BC, they may very well NOT be able to experience similar things together. On the other hand, them being the same age or only a little younger, would make it very hard not to compare - and comparisons would likely always turn out unfavourably for the AC. Which might just reinforce their belief, which they gained from early experiences, that they are rubbish, not worthy of being loved, not as good as other children, undeserving...
- AC having more needs, and the effects on sibling rivalry
I think it only makes sense to expect that an AC will have a lot of needs. So imagine your two similar age children, maybe both are at infant school. You are constantly going to meetings, therapy sessions, doing one to one work, ... but only with/for one of the two. The other, though at a similar stage, has to get on by themselves as you simply don't have time to attend to their needs as well. Doesn't sound ideal, does it?
A larger age gap means that whereas the AC will have more needs, they will be of a different nature than the needs your older BC has. Your BC won't get (as much) jealous of you taking the AC to regular SN toddler groups, when they themselves are at secondary school.
Your BC will have to share, not just toys, and not just for a few hours a day... they will have to share YOU, and ALL THE TIME. So if the kind of attention they need from you, is very different than the kind of attention the AC needs from you, there will be less competition and less comparison, and hopefully, more getting along.
FWIW my personal hunch is that the younger the children involved are at the time of adoption, the less problematic a small age gap; and the older they are, the more necessary a large age gap. So for instance, if you were adopting a 5yo I'd think a 4y age gap would help, if you were to adopt a 7yo, then a 7y age gap would be helpful, but if you were to adopt a 18months old, then a 2y age gap may be 'sufficient'. But that's just a hunch.
I have been interested in this question myself, and have frequently heard that 'all research and experience says that the larger the age gap, the better' however no-one has been able to actually point me towards this research, and though I am a social scientist and have access to research publications, and know how to search databases, I haven't found anything myself either. I'm not saying it isn't true, only that I haven't found it yet. And though there are IMO some 'bad' arguments for age gaps, that doesn't make the 'good' arguments less true.