Regardless of weight, I would wait to ride her personally until she is 3 and her back is stronger. I am like.. 128 or so pounds and I wouldn't feel comfortable sitting on her yet. She's just not ready, in my eyes.
Once her back is strong for a rider, in my opinion, you could easily ride her a handful of times and when she's really strong and has started to build up some muscle you can start riding her more frequently as she'll be able to support herself without any trouble :)
Now for just getting on her and walking a little bit, I think that would be okay!
"Strength is the ability to use a muscle without tension"