I think it's a myth, the same as saying that all women who hit 40 are going to suddenly become "mutton dressed as lamb".
I think we all reach a time in our lives when we might look back and wish we'd done....been... whatever, and then decide that it might be a good idea to do it before ill health or other committments take over.
Having said that, a close relative of ours got to 40, and suddenly swapped his car for a bright yellow mini. He's a big bloke, and looked pretty silly squashed into it, with "Born to be Wild" blasting out from the speakers! Suppose you could say that this was his "mid life crisis", and at least he didn't start dressing diferently or wearing different aftershave - otherwise his wife might've thought he was having an affair!