It's partly jealousy
It's also that to many people, something "different" isn't something they like. The norm for relationships is no more than a couple of years age difference (Hardly surprising, when people meet at school, or in the same places which appeal to their own age group). As a larger age gap is "different" - they feel they have to poke their nose into it.
And it is also that many people are simply armchair experts. Yes their marriage to Shaz from accounts might have fallen apart when she was caught in the stationary cupboard with Kevin from sales, but that doesn't mean they don't feel they shouldn't cope on something they "just know" won't last.
Sheesh!
