Remembering how old you are is one thing. Remembering where to put (or not put) the hyphens when you describe your age is another.
A thirty-one-year-old woman should be able to write grammatically. I am thirty-one years old. A thirty-one-year-old is old enough to know better.
Did you see those hyphens (or lack thereof)? They’re all correct. Do you know why?
Here’s the reminder: if the age is being used as an adjective or as a noun (as it is in my first and third examples), use hyphens; if the age is part of the adjective phrase following the noun (as it is in my second example), don’t use hyphens. Got it?
When a child is two years old, he or she doesn’t care about grammar. Maybe a ninety-year-old still doesn’t. Either way, knowing the rule doesn’t hurt, right?