The point of this film, although not necessarily obvious, is that love is not about age. It is about having a healthy, trusting and accepting relationship, strong enough to last for as long as it can. If both parties to such a relationship happen to be of equivalent age, that would be conventional, but equivalent age is no more essential to such a relationship than being of the same race, gender, or some other superficial feature. What difference does it make if one of the parties in the relationship is significantly older or younger than the other, so long as both are comfortable with that arrangement? To argue that differences in age matter more than differences in race or gender, is merely to engage in another form of bigotry.
On the other hand, of course, to "prefer" someone in a relationship because of their age, race, or gender, represents as much bigotry at to "exclude" them for such reasons, and that's where this film delivers its message. Harry "prefers" younger women, while Erica "excludes" younger men. Erica can't handle a relationship with a younger man because of that difference in age, while Harry can't handle a relationship with Erica because of their equivalency in age. As a result, the question of whether age, as a discriminating factor in seeking a healthy, productive relationship, is worthy of serious consideration, becomes central to the message of this film. It does not suggest that differences in age are preferable, only that differences in age are not the point.
On the other hand, of course, to "prefer" someone in a relationship because of their age, race, or gender, represents as much bigotry at to "exclude" them for such reasons, and that's where this film delivers its message. Harry "prefers" younger women, while Erica "excludes" younger men. Erica can't handle a relationship with a younger man because of that difference in age, while Harry can't handle a relationship with Erica because of their equivalency in age. As a result, the question of whether age, as a discriminating factor in seeking a healthy, productive relationship, is worthy of serious consideration, becomes central to the message of this film. It does not suggest that differences in age are preferable, only that differences in age are not the point.