Hi there,
We don't have a lot of post-bottom surgery guys on the board, but I'm in some other support groups for people having bottom surgery so I can speak generally about those experiences.
It seems like once everything is settled (completely post final stages, no lingering complications), people are happy with what they've got. It tends to be the complications that get people down, but once those are cleared up, it seems like guys feel pretty great.
As far as how long, in the US the requirement is one year on T/one year real-life experience (so living as male) to be eligible. The majority of guys are getting it covered with insurance now as opposed to paying out of pocket, at least in the US. Speaking personally, I'll have been medically transitioning for about a year and a half by the time I have phalloplasty. My cost with insurance is $6k for a hysterectomy, vaginectomy, and phalloplasty with all the extras (urethral lengthening, scrotoplasty, glansplasty). It will be up to another $6k at a second stage to have an erectile device and testicular implants placed.
I'm 26. I can't really give you an average age, because it really is all over the place. Guys are getting stuff done much earlier now, but older guys are also getting access to trans-inclusive insurance for the first time, so I don't think there is an average.