"If you’re not a liberal when you’re 25, you have no heart, and if you’re not a conservative by the time you’re 35, you have no brain" is just another way of saying "I have no morals and think people who do are stupid suckers." When someone is telling you who they are, listen.
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I had a talk about this with my conservative parents, saying "isn't it a little bit hypocritical to switch over once you start making money, do my friends benefiting from free tuition stop mattering?"
If someone is telling me something like "If you’re not a liberal when you’re 25, [..]" to my face I usually just ask them to explain to me in detail why it is so. Sometimes it even helps a little if they listen to themselves trying to defend this position.