I actually always thought the same thing. For all of the 20-some-odd year olds that are Lex's sons, Lex doesn't look all that old!
The actor John Shea (who played Luthor on L&C) was born in 1949. It would make him 45 years old when Lex almost married Lois.
In this pic (from
Gossip Girl ) he's 62! He sure looks good for his age.