What really makes the limit 18 years old in relationships?

For example, they say it is 18 legally. But some people are still childish at 18.

As a 34 years old man, my limit should be at least 25 years old girl. Women who are more little than that don't sound mature to me. But some people may prefer different.

Like if a 20 years old guy be with a 17 years old girl, is he a pedo? I mean who put these limits, and what limits are really safe?

What really makes the limit 18 years old in relationships?
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