When he was all of 23.
The song, written by Joe South ("Games People Play", "Walk a Mile in My Shoes"), spent 3 of those weeks in the top 10 (10, 10, 9) in August 1965.
Billy Joe Royal, Singer, Dies at 73; His ‘Down in the Boondocks’ Was a Hit. (The New York Times, 10/7/2015)
- "Once in a while I hear it on the radio, and it still stands up. The song meant everything to my career. I was making about $125 a week before that.”
- “When he [Elvis Presley] made it so big, all us Southern boys thought maybe we had a shot, too."
- “I say this with all humility. The old voice has stood up, you know. [snip] The other thing is, this is all I’ve ever done. I had to stick to it because I’ve never done anything else.”
Royal recorded another Joe South song the following year, but it stalled at #52 on the Hot 100 in the fall of 1967 and dropped out of sight after 8 weeks.
Giving it a little more heft and endurance (4:11 vs. Royal's 2:28), Deep Purple rode it into the top 10 for a 3-week visit (8, 4, 4) in the early fall of 1968.