I find it extremely odd that a 20y.o woman (who in most cases is barely a woman) and a 45y.o man are in the same space emotionally, mentally, physically, and spiritually. Usually my answer would be, let the 20y.o live her life, and enjoy doing things 20y.os do,you've had your fun, allow her to have her's. Eg, my friend started dating someone, also in his 40's when she was 21, at first everything was fine, now they're in trouble coz at 24, she feels like she hasn't lived, now the poor man is stuck getting up in the middle of the night, driving her to and from nightclubs and house parties with her peers, or he risks losing her if he expresses his distaste for her new lifestyle... BUT, THERE ARE EXCEPTIONS TO THE 'RULE'... If this is one of those exceptions and the 2 of you are at a similar place in your lives, why not?
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Goodluck