Well now isn't that up to the individual, the 20 yr old to decide whether it's right for her and not you or the rest of society? Really quite honestly what's fair and what's not fair, who's to say? Let's say this 20 yr old finds another 20 yr old, would there not be challenges involved there and burdens that come with all relationships? it's a matter of choosing what burdens you choose to take on and whether the partner you choose is worth all of it.

As an older partner, choosing a younger woman (or man) has it's unique burdens too, but isn't that also up to the individual person to decide and not for outsiders to say whether it's fair to him or her to get into the relationship?

If you're a 50 yr old and you feel you shouldn't date anyone in their 20s or whatever age range you want to limit yourself to, that's perfectly fair for you but the problem arises in generalizing your rules for yourself and making them as deFacto rules for others. Don't date younger women, your rules and restrictions are yours. Would it be fair for me to tell you that it's not right for you to date someone that is say, your age? Should I tell you that you SHOULD date younger women? no because by the same token it's not for me to say.

If two people care about each other and are willing to take on the challenges that the relationship might bring, that is all that matters outside of someone dating someone illegally young.