Related Clues:

• Energy needed, working for a long period
• Wise men having no leader for years
• Single European taking top position for a long time
• Long time no see — finally turned up
• In space, one thousand million years
• Period too long to measure
• Isn't getting any younger
• "Forever"
• Becomes ripe
• Long time periods
• Ages and ages, collectively
• '___ 8 and up'
• "Rock of ___"
• The 13 of PG-13 and 17 of NC-17
• Seasons, as wood