Yes, adults can ride ponies, even Shetlands. However, no horses under 3 years old should be ridden as they are not yet completely physically developed by that age and ridding them may result in back injuries and deformations. It is recommended that you wait until your horse has reached its full physical development before riding. The Shetland pony is not as "small" as most people think and they can be pretty strong too. In fact, the Shetland pony is a fairly large breed compared to some of the other breeds in this group.