Lots of experienced shooters on here ..not just me ...but sure, I'm around.
Older BT-99's vs newer ones ....not really much difference in my view - at least mechanically. The BT-99's were made from late 1960's thru 1995 or so / then in 2001 they went into production again and continue today. At some point - they started putting adjustable ribs on them / and some add on systems like GraCoil, etc ...( probably in 2001 or newer ).
There was also a single barrel model BT-100 / and its significantly different - in that, its trigger group is removeable by moving a lever on bottom of trigger group and it pops out for easy service and cleaning / but it was more expensive to mfg, and they were only made from 1995 - 2001 or so. The removeable trigger is a nice option / if you can find a BT-100.
But a single barrel break open gun ...mechanically, they're all about the same. Most are 2 3/4" chambers...and barrel lengths are common in 30", 32" and 34" ....and there are some differences in the forend styles. Some have adj combs / some don't. So you have to look at all those features and decide what you want. In terms of durability - I don't know of any issues on the BT-99 or BT-100 prior to the new generation of BT-99's ( where with the new adj ribs / some shooters say, they can't get the point of impact down far enough / they shoot too high - and they are a little fragile if you aren't careful in and out of gun racks - in terms of the rib adjustability. But adjustable ribs are the newest thing in Trap guns ( Krieghoff, Ljutic, Browning, Caesar Guerini, etc - on their single barrel guns ).
Many of the older Brownings ( had the older Invector chokes in them / they're shorter). Browning started putting the Invector chokes in shotguns in early 1980's. When Browning started back boring barrels - in late 1980's or early 90's - they changed the chokes to Invector Plus. Most guys will tell you they think the Invector Plus guns ....pattern a little better. If the gun you're looking at was made in the 1970's - its probably a fixed choke / likely a Full choke - so no, I wouldn't recommend any fixed choke gun vs a changeable screw in choke system.
In general, I think the Citori XT Trap over under in either 30" or 32" barrels is a more versatile shotgun - you can shoot some sporting as well as Trap singles or doubles ...or continental Trap - where you can take 2 shots at one target. I'm not saying a single barrel break open Trap gun is a waste - but its a "one trick pony" in that its only good for Trap singles. I especially like the Citori XT Trap in 32" barrels.
In general ....a heavier ( between 9 lbs and 10 lbs ) and a longer barrel - like 32" in O/U or 34" in a single ...is a plus for Trap. Heavier weight helps on follow-thru / longer barrel gives you a better sight plane. But its a balance issue / what feels good in your hands vs my hands / I'm 6'5" and 275 lbs ...and while I like a 10lb Trap gun / its too much for a 13 yr old ( its too heavy / too clumsy ) ...but they may grow into it. Kids grow so fast / change so fast - its hard to predict.
Even my general purpose guns ...for skeet and sporting ...in 12ga or even in .410 ...I shoot Browing Citori O/U's ( XS skeet models with 30" barrels ). My boys / my older grandkids ( 15 - 20) - are all shooting 30" or 32" over unders now ...for sporting or Trap or Skeet ( in 28ga, 20ga or 12ga). Some of the younger girls / like 13 - don't have a lot of upper body strength - but I picked up a Benelli super sport model, 20ga, in a 28" barrel ( 6.2 lbs ) and its been a nice gun for the younger kids to shoot. I also have a 28ga O/U ( XS Skeet with 30" barrels ) that some of the kids were shooting at 11 and 12 yrs old ....
so part of this is picking a gun ...and part of it is managing the recoil ( loading lighter shells ) ...and making it fun for them.
I like the BT-100 over the BT-99 ...but I like it better in 34" than in 32" / but I don't like it nearly as much as the Citori XT Trap in a 32" ..../ and I would not buy a gun without the screw in Invector chokes / and I'd prefer the newer Invector Plus chokes ( my BT-100 has Invector plus in it ).
Is a 26" or 28" O/U too short for Trap ...no ! Is it optimal - maybe ---but that's partly why there are so many guns out there / what fits me, isn't necessarily your best option ( size of hands, etc / lenght of pull / drop at comb /drop at heel / parallel comb guns ...) lots of options...
Hope this makes some sense...