There are no set numbers. How is your health now? Been told that you are pre-diabetic, or borderline hypertensive? What about family history? How much do you plan to gain? Any sore joints?
Can't have a set answer for that.. everyone's different. It's like coronavirus:age is a big factor and the older them the more likely to be serious, but that's a big generalisation and as specific as you can be.
more and more people postpone their gaining until they are older now, which makes sense to me (children moving out, more financial stability, slower metabolism, nobody is expecting you to run a marathon at a certain age) ... if it is something that you always wondered about, I'd say go for it, better now then when you're dead!
I'm old, mid-sixties and I'm a causal gainer. I did weight until I was older for several reasons, some mentioned here already. My main reason was retiring from the military reserves. The military has fairly strict weight and appearance standards, which held me back for many years. Once I retired, seven years ago, I have been gaining, slowly, but steadily gaining.