What might make someone develop an allergy to shellfish later in life?

Unknown. Many people (at least half) develop shellfish allergy later in life. This probably reflects the way the immune system develops, as some allergies are more common in very young children than adults (like egg), while others develop later (like pollen, and shellfish). The exact mechanism is still unknown though.
Unknown. See your allergist for testing. Oftentimes people are allergic to only one or two items in a particular class of foods and you might be tolerant of others in the same family.