Welcome! And feel free to ask all the questions you want -- I've been a horsey nut all my life and I still learn new things here, so there is a great group of generous people to help you into the money-evaporating, oh, I mean horse-loving world, LOL!
Never let age stop you -- and at your weight, you can certainly start lessons and you don't need enormous tall horses, just a stockier built horse with a shorter back and thicker leg to help carry the weight a little better than a narrow skinny thoroughbred or something like that. As you learn to balance yourself better, you will find yourself having less impact on the horse too as you stop bouncing around and find your body control, so that is always an exciting progression. And lessons are killer exercise too so if you've been wanting to get more fit, a good lesson program will definitely help with that too.

Solaris -- 16 hh Appendix Quarter Horse = MY DREAM COME TRUE!