Why does Belle fall in love with Rumple? Is age not an issue?

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

And concerning the specific question of why Belle fell in love 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 more common.

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