according to Imdb.com, a Canadian actor called Bruce Ramsay played the part of Young Joseph Bonanno, aged 17-27, in the 1999 TV-movie "Bonanno: A Godfather's Story" He was born in 1968, so this means that he was 31 years old when playing a teenager.

Now I have to admit I have never seen that movie, so I don't know if he pulled it of. But if he did it leaves some options open to actors for playing the parts of young Sonny and young Hagen in the never-to-happen-Godfather 4.

Personally, FFC should have recruited young, relatively unknown (Italian) actors with satisfying, realistic ages to play the Corleone youngsters, and a GF4 shouldn't tell to much of the known characters in their younger years, but should introduce some new big shot bad guys, for whom age and looks don't matter. I'm thinking FFC could invite his family again, with Nicolas Kim Coppola leading the pact (a guy we know better as awardwinning Nicolas Cage).