Let's put this in simple terms.

The reason this belief exists in your head is because society is of the opinion that older people are more worldly and experienced, meaning that their social and worldly knowledge is somehow taking advantage of the younger person.

The answer really lies in, are you cool with the idea of using social experience and knowledge to take advantage of other people? Consciously learning things from these communities is contrary to this bit of social conditioning.

Personally I think it's all bullshit. There are fucked up and socially stunted people in all age groups, whether you are taking advantage of a person's lack of experience merely because of your own experience is only identifiable on an individual basis because there are plenty of socially savvy and experienced people at 20 compared to 30.

Define taking advantage of someone. When you can define that you can draw the moral lines accurately here.

Personally I think it all lies with your own behaviour and the intention of your interactions. If you are acting honestly and being real opposed to a trickster using manipulatory tactics in order to get a girl then you are perfectly in the clear.

That girl will be approached by 30 other guys in the same night who are in her own age range, many without the level of morals you have shown by even considering this social dilemma. She can do much worse.