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When you are younger, the age difference means more. Having a partner who is up to ten years older can be fine. Even with ten years difference, each member of the couple will notice important differences as a result of age.
Once you get to differences above ten years, you start getting into parent-child dynamics which can be unhealthy for the relationship. To be safe, keep the difference under ten years. Even when you do have an age difference and you are in the relationship, make sure each of you feels you can openly talk about how the age difference plays out for each of you in the relationship.
The truth is that age really does matter.