Possible Clues:

• Come __ (mature)
• 18, say
• Eligible to vote, say
• Old enough to buy beer, say
• 18, often
• Having acquired door key, maybe I'll turn to check what's following me
• Twenty-one, say
• 18 or 21, say
• Minor no more
• No longer too young to work hard in Rome (odd parts excepted)