Hi, I hope a minority voice is welcome on the board too.
I have 2BC and 2AC with less than two years between BC2 and AC1. Yes, my children are probably too young for me to really know/comment, they haven't reached adulthood yet, my eldest is under 9, and I've only been an adopter for four years, but from my (limited) experience getting a good match is far more important than getting a specific age gap. There's six years between my eldest and youngest and actually I find that age gap the hardest, as they're worlds apart in what they do, how they think, and what they want; they never really gravitate toward each other, there's nothing to unite them (yet) (except for tickling, and being noisy).
Yes, adopted children are traumatised (though this manifests in many ways, not just violence), and my pair have various issues (as do my birth children), but I'm not so sure a big age gap and keeping children apart what socially?physically? emotionally? promotes a 'better' relationship just by default. I think its more complicated than that, and based on personalities and the needs of all your children, and also maybe taking into consideration what you're like as a parent/ or parents. Can you cope with chaos? have you got the energy for 2 under five, or two under 7 for that matter? Now add in the higher degree of unknowns with adoption. Think practically too, do you, or will you have the time, to listen to one child practise their phonics (say a 5 year old), while a toddler (with speech delay, so therefore quite frustrated) runs about? Zoom into the future, and I don't know the answer to this myself yet, but can you cope with two teenagers at the same time? (I'll have four teenagers at the same time, so I hope I can). (OP, you already have a teenager so you're better equip than I was when adopting to answer this).
You're welcome to PM me so I can elaborate on why I'm finding close ages gaps positive, but in short; school, friendships and alliances via common references, less of a distinction between 'adopted' and 'bio child'which I think is helpful, others might disagree.
And as a snapshot to my day, my very confident DC3 (that's also AC1 in this post!) spent her day helping shy DC2 (aka BC2) come out of his shell at a half term activity camp. DC2 had a panic before bed, that he might be put in a different group to DC3 tomorrow.
Re. agencies and matching - we were matched very quickly, BC were seen as a positive and age gap didn't even get a mention at panel.