"It's weird to think about it. My mom and I joke about it all the time," ... "She says, 'I'm still not retired, and you're going to retire before you're 30." -- Michael Phelps [Link]
He is probably the only US Swimmer in history who could even do that. Even Johnny Weissmuller had to become Tarzan before he could retire.
I would also like to add that with colleges cancelling swim programs, governing bodies doing what they can to keep swimming from becoming commercialized with suit products, he may be the last one to do so as well.