Can a gluten intolerance have an onset later in life (starting in your 20s) or is it more typical that one would have an intolerance from birth?

Acquired: It is almost always acquired, and the 20s is a common time.

Never from birth: The body has to first be exposed to a protein (in this case gluten) before it can develop antibodies against it. This may take place after the first exposure or after the 10, 000th. So, one never sees it from birth, rarely in infancy, but commonly in children and adults.