He was young (scooz pun) when he joined.
I think he's incerdible, seen a good few AC/DC gigs and he's soloing and on the go the whole gig through. I used to work in Hammersmith Odeon (as it was then) in the '70s and, with early radio 'lead' was taken through the audience on the ground floor on Bonn's shoulders, left to run around the foyer, up the stairs and round and round the bar and back out into the auditorium circle, soloing the whole time without missing a beat. Legend.