If you think about it with vampires and other creatures that live on for a long time, age is a different concept to them. The monsters of SecuriTale are much the same way. Sans is waiting for Frisk to be ready for him, both physically and mentally. In regards to what's important to him, first and foremost is her happiness, which means that she must be both old enough to be in a relationship as well as mentally able to accept him. She is, as of the moment, not ready at all for anything as she can't even love herself. Sans himself however loves her, and would like for her to be with him someday.

I know ST walks a really sensitive line, but it's something important because the love the characters have for each other is what governs their decisions.