Do you think I should get my mom to leave my dad?
I'm not sure, why do you want your mom to leave your dad?
He's 20 years older than her and I think that's too big of an age difference.
Age isn't necessarily a barrier to happiness in a relationship. Your mom may be happy and unless you have other evidence you should leave them alone.
I'm looking out for my mom and I'll do what's best for her.
I think you should let your mom decide what's bets for her and what makes her happy. If she's happy, just leave her alone and let her continue to be married to your dad.