I see that this subject is still active so I want to add this.
Can adults ride ponies? Sure they can, but we have to relate the issue to the size of the pony and the size of the adult.
First up – certain pony breeds are stronger than others. Take the Welsh Cob which is pound for pound super strong. Another is the Icelandic which for some strange idea is being referred to as a “horse”, while in fact it’s a pony sized mount.
Secondly – there are small and very light adults.
So, according to the US Cavalry manual of old, the equation is, the rider and tack (yes that does include your huge Western saddle or whatever) not to exceed 20% of the horse’s weight. But that’s the absolute max.
Fact is that the horse or pony, will preform better if the equation is more around 10 % of the horse’s weight, give or take.
So, a large pony of a strong breed like a Welsh Cob, Connemara or such is most capable in carrying an adult. But we need to consider the weight of the adult.
I found out that the average weight of a New Forest Pony, comparable to a Welsh Cob, is 300 – 500 Kg or 662 – 1103 lbs. That’s a huge variation. Now for simplicity sake lets calculate that the one we’re looking at is 800 lbs. At 10% the rider and tack should weigh no more than 80 lbs. At 15% that comes to 120 lbs and at 20% it comes to 160 lbs.
Just food for thoughts and nothing set in stones. I have myself as an adult ridden and competed on Welsh Cobs and at the time I weighed in at about 128 lbs plus the tack. I can attest the ponies/horses had no issue with my weight nor did anyone ever question the pony’s ability to handle my weight.
Still, my feeling is that these are judgments needed to be made by those capable of doing so.