How much age difference you consider not advisable in a relationship?

In this case the man being older than the woman. Even if they are adults? How about 15?

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Ok so lets change to this case is about a Hollywood actor who is 44 (but physically looks like 50) dating a 29yrd old woman. 15 years difference (no is not Leo Di Caprio is another actor). Is she his sugar? Hehe!!!
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The actor in question is Chris Pine, his lady is 29 with a baby face genetically and he is 44 but physically looks so much older. Now they are the couple of the year
How much age difference you consider not advisable in a relationship?
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