Pro boxing or boxing at all? Matt Skelton started boxing in his mid 30s, Rigondeaux started pro boxing at 28 for example.
Erislandy Lara turned pro about a year or so ago.... Gamboa.. Tony Thompson Audley Harrison Rigondeaux
Ron Lyle is always my favourite example. Another thread asking the same question that mentions some others: http://www.eastsideboxing.com/forum/showthread.php?t=166498