Bevis's Birthday

To kickstart the week, a little algebraic puzzle I adapted from this week's "Ask Marilyn" column in the Sunday Parade Magazine:

Suppose Bevis's birthday is this month (he's an adult), and on his birthday he will be the same age as the 2-digit year in which he was born (i.e. 1940, or whatever). How old will Bevis be on his upcoming birthday?
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answer:  57 years old