Join Date: Sep 2011
Location: In Sunny, HOT and HUMID S.C.
As already said over and over you are not to big for those horses.
I'm going to guess that you've asked the question because of the horses age.
While they are both in some stage of the "Senior" age range if they are in good health and fit there's no reason why you couldn't still enjoy riding them. You are certainly not big enough for your size to cause a strain on them.
I wouldn't recommend doing an endurance run, but a pleasant hack should be fine. I've known horses to die in their early 20's and I've seen them live over 30. One of my cousin-once-removed had one of his recently that was over 30, but that was 3 years ago, so I don't know if it's still alive. While it wasn't worked hard or a lot, it still did light work like operating the cane grinder and pulling a small cart with things from the garden.
It really depends on the condition of the horse. Just like you can find people who run 20 miles at 60 while others can't run 2 miles at 40.
They're always going to be bigger and stronger so you better always be smarter. (One of my grandfather's many pearls of wisdom)