I just had a thought:
If I get an SPG later this month that backdates to 1/12 and cancel my stupid, useless Gold card (opened 7/12), I end up adding six months of age to my accounts.
I can extend this further in the future. Get another Amex in 1/14 that gets backdated to 1/12, cancel my Costco TE card opened in 8/12, gain 7 months. Repeat for the Amex Hilton, opened in 11/12, new card backdated to 1/12, gain 10 months.
I gain an average of 7.6 months for each card I strategically cancel.