The prime is the peak of your power.. Hashirama said now they are brought back with their full power, which is limited to the age they died at.. if hashirama died at 80 , he'd be brought back with his full power when he was 80
Hiruzen isn't in his prime state... the average would be somewhere in your 40's I'd guess,balance of speed, strength etc...Tsunade herself said she isn't in her prime in part 1.. and she was 51 then?....
That average isn't always true... Danzo for eg, prime was his state in p2...after implanting moukouton, shishui's eye/arm and all those other eyes.... that was when he was his strongest