Not 20 years, but more than 10.
When we were younger it didn't matter. We both had the same goals - home, family, kids - and same tastes. We enjoyed the same things. Age literally never came up, other than in idle musings about our career plans and retirement ages.
Thing is, as the older one got older they became more... 'old'. Sensible. Didn't enjoy 'frivolous' fun things anymore, didn't enjoy going out or restaurants or days out. No longer wanted to ramble in the hills or even watch movies. They wanted to work and only wanted to converse about pensions or the stock market. Talk turns to retirement and 'oo my aching bones'. Which is a joke really from someone not yet 50 :p
The younger one still wanted to enjoy life; they are not ready to go all pipe-and-slippers. They enjoy travel, days out, exposing the family to new places and experiences. It is not yet, for them, time to retire. Having had children young for the sake of the older one, they are eager to enjoy what is still, frankly, their youth. With their beloved partner in tow. Who doesn't want to leave the house.
That gulf causes rifts now. One adopts the mantle of 'dull', the other of 'infantile', and it feels like there is no common ground. Both have a point. One "already did all that nonsense" as a young 'un, and the other... did not.
At 20 and 35 you might watch the same films, listen to the same music and enjoy the same activities. At 35 and 50 you may not. The gulf widens.
Seems easy at first. Couple o' decades down the line, the cracks do begin to show.