Is it forbidden to lie on a website about one's age to create an account—for example, if a 12-year-old says they are 20 years old—especially if the website prohibits minors from creating accounts?

The basic rule concerning lying is that it is forbidden and only permissible when there is a necessity to achieve a benefit or avert harm. This includes lying about one's age, which is not permissible unless there is a need for it, especially if the prohibition of creating electronic accounts for those who have not reached the permitted age serves a beneficial purpose.