Just some anecdotal data, it can definitely start before 50. My brother was in his late 40s when he started showing changes (not necessarily symptoms). He was 48 when he quit working (I think he was actually fired). My sister is older, but appears to be in the early stages (not remembering things from earlier in the same conversation, needs help making decisions). I'm 50 and starting to have memory lapses.
Definitely don't wait until 50 to look for symptoms.