Mona Chalabi looked into the average age difference within couples:

• Males are 2.3 years older than the female in the average heterosexual coupling.
• In 23% of couples the female is older than the male.
• While there are no official statistics, data from Facebook indicates that in homosexual relationships the average age difference increases as the couples get older.
• This is true of heterosexual relationships as well, although the effect is not as big.
• For example, for a couple who has an average age of 20, the average age gap is three years for homosexual relationships, and two for heterosexual ones.
• By the time the average age of a couple hits 50, the age gap averages around seven for homosexual couples and five for heterosexual ones.

See a chart showing the correlation, and read more details here.

Source: FiveThirtyEight