What will happen if I will purchase a plane tickets being under 18?

I basically agree with Sheik Paul of Osawatomie's answer, but I would tailor the response more closely to the question. Therefore:

I could send my money to my parents bank and let them purchase it for me but in my opinion it doesn't really matter. Either way they cannot see my age when I purchase something

As others have mentioned, the point of the age restriction has to do with the legalities of the financial transaction and, probably, the contract created between the purchaser and the seller. It does not mean that people under 18 are restricted from traveling, only that they are restricted from buying tickets.

and even if my parents buy it I still could get in trouble in airport since I'm 16.

If your parents buy the ticket, you can't get in trouble for buying the ticket. It is not forbidden to purchase a ticket for someone else, and if your parents have bought the ticket there's no trouble for anyone to get into.

The age restriction does not apply to travelers; it applies to purchasers.

So I think there's no point in sending money to parents since it will do nothing. So the whole 18 years requirement seems useless to me.

As outlined above, having your parents buy the tickets is precisely what you should do. The fact that those under 18 have a limited ability to enter into legal agreements may be useless, but the line must be drawn somewhere, and that's where it has been drawn at the moment.

If you are in the USA, you can have your parents buy you a ticket as an unaccompanied minor.

Although each airline is likely to have their own policy regarding how old you have to be, if there is a surcharge or not, special accommodations, etc.

This is likely the only technically legal way to fly to London (assuming from the USA) without a parent or other adult.

In reality I am not sure if the airlines would care about your age, but there is always the possibility of an ID check when you check bags or go through security which could raise questions.