25-30 would probably be easiest, especially if your mental age also freezes at the time (though neuroplasticity might be a boon).
Assuming my mind stays sharp, able to learn and not as stupid as sub 25 then yeah back to 25-30. Age is more attitude than anything else and makeup can do a lot for you as well so would not entirely suck to be 15 -20. Below that would be a bastard really. Above that assuming it is just grey hair and wrinkles then all good really, an eternity with an aching knee or something would be less fun.
Not to mention if I am going to kill people to assume their identities a 20 something is in many ways a lot easier to disappear than finding a 50 something without community connections that could be exploited like this, and the "good old boy that never got a passport/photo ID" you can drop a couple of grand on pool is getting smaller by the day and only gives you a few decades anyway. Or if I am farming humans to assume identities (let them grow up, no ID during school, move away, dispose of them/have a "servant"...) then also less time to create.
Also yes I have debated what I would do if I was eternally fixed in age to allow myself to blend in. If I somehow had money to play with (a possibility if you have a lot of time) then I could see having a cutout or stand in be a business and live off the fat of that.
