Legal in what respect ?

Well, really, it depends on where you live, and it being accepted in your community is a different thing entirely.
Anymore information you're okay with sharing? Maybe someone could better serve your needs then.

Google age of consent and age of emancipation laws for the state you live in.
If you are supported by a parent/adult who is opposed, the 26 year old could be charged with various crimes, so be well informed.

Since your question was whether it is legal I've moved this to the legal forums.

ANY question on law NEEDS to include your general local as laws vary by area.

There is also the issue of what you mean by "to be with..." There are no laws that govern dating, but there are laws that cover sexual activity. Also being a minor there are laws that govern interfering with your parents.