Why does Belle fall in pag-ibig with Rumple? Is age not an issue?

There are a lot of arguments against their relationship, but on this ipakita age isn't a good one, unless you're willing to say the same about Hook/Emma and Neal/Emma.

And concerning the specific tanong of why Belle fell in pag-ibig with Rumple, according to her she saw the man behind the monster.

The age isn't really an issue. They were both adults when they met. Couples with a large age gap are rarer than couples who are closer in age, but there isn't anything wrong with them. In fact, they used to be madami common.

In short, she fell in pag-ibig with him because she saw the man behind the monster and wanted to fight to bring him out. He began to treat her madami kindly while she was working as his servant, and they both fell in pag-ibig with the other.