What are the down sides of being friends with someone 20 years older than you if you're about 20 yourself?

I desire to be friends with someone 20 years older than me but I don't think they feel the same. I'm trying to get over it.

Tell me the down sides of being friends with someone twice my age.
Update: @Anonymous, such a cold answer. Have you never been attached to someone you really like before?
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