The 29-year-old is best known for his tracks such as Down and Do You Remember and said he is surprised at the new breed of singers who are getting younger and younger.
He said to UK's X magazine: 'It really does make you feel old. With Bieber, I consider him a friend as we've done shows together, but then sometimes I can't help but feel like his dad.
'I'm like: 'Damn, I'm ten years older than you. You were, like, five when I had my first girlfriend.' It's just crazy. But it's good to start young so you can hone your skills, then when you're older you'll have this game locked down.
'I've talked to Justin Bieber about this and I'm like: 'Get in the gym now, practise the skill of the gym, you'll be buff.''