I would go for the 4 years and older but not older than 6 years.
That means that the girls must be older than the guy at least in the age group 18-26 because girls are about 2-3 years more mature than guy at the same age.
If an 18 year old girl dates a boy of the same age, his maturity is more likely the one of of a15-16 year old child and that is definitely not ideal. As an adult girl, you don't want to have a child as a partner.
The gap gets smaller after the mid twenties but still, a gap that is more than 5 years is not ideal in my opinion.
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