How can a man be more assertive with an older woman?

How can a man be more assertive with an older woman?
Maybe I'm being wrong in putting this on the age, but I've noticed that I manage to be more assertive with women who are younger than me - for example the 19-20 years old student who's helping me out with some projects - while I have noticeable difficulties with my fiancée, who's 26. Granted, some of this can be attributed to her having always been the "alpha", even when we were in school years ago, but I've had similar issues with my ex who was 24 at the time I was 21, and some other women I've dated who were around that age or older.
So, is age really a factor?
How can a man be more assertive with an older woman?
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