What do people think about appropriate / okay age difference (for relationship partner)?

I heard somewhere that psychologists say that 10 years is 'significant' (though who knows what it signifies lol).

Talking about consenting adults here.

But with respect to consenting adults (and the absense of inappropriate relationship due to therapist/client or doctor/patient or supervisor/student etc) what are peoples thoughts on what kind of age difference is ok.

How about... 15 years?

Does it matter more how well you get on and where you are headed etc maybe instead of age? Or... Are there limits?