Welcome ✋
Good starter guns can be had from CYMA and SpecnaArms for around £150.
Most guns around that price are decent enough to start with but I would stay away from Nuprol stuff in general.
Yeah, there are loads of posts about first starter guns and kit in general.
Keep some budget for decent boots with ankle support and eye protection. ( would suggest to look at posts about the best eye-pro)
You'll get the hang of things soon enough ?
Don't worry about age since you can play at your own pace. (I know players in 60s and 70s that play)
But don't expect to stick with your son ?