Related Clues:

• Cultural periods
• Atomic and Ice, e.g.
• Middle and Stone
• Periods
• 50/50
• Levels off
• Wrinkles, say
• A really long time
• Long period
• Mellows, as wine
• A very long time
• Odds partner
• Shakespearean seven
• Adds one to three, e.g.
• Shakespearean septet