How old was Brian Johnson when he joined ACDC?

Can you tell me, approximately how old was Brian Johnson when he first joined the legendary rock band AC/DC? I'm curious to know if he was already a seasoned musician at that point, or if he was still relatively new to the industry when he became the lead singer for one of the most iconic bands of all time. It's fascinating to think about the timeline of his career and how it intertwines with the history of AC/DC.