So, yes, its perfectly acceptable. Im 41 and my best friend is 27. I have another 2 friends who are older than me, one is 55 and the other is 62. Makes no difference how old someone is.

Does age matter in friendship?

In close friendships, age peer relationships were experienced more intensely than age discrepant relationships. Feelings related to companionship, satisfaction, intimacy, nurturance, and reliable alliance were significantly greater in age peer close friendships.