Why is my age bothering him?

We're friends and he is 9 years younger than me. Whenever the talk comes to age, he teases me about being old. Sometimes really mean, I'd say.

I never asked him to be my friend. From the start, he was the one who wanted to get to know me and was trying hard to make us become friends. He always says how we'll be friends for life, so to say. At the same time, he isolated me from his family and friends of the family. At first, he insisted on me meeting all of them. I respect that. We have a mutual group of friends and we hang when we're with them. It doesn't bother me, just trying to understand it. He doesn't have to be my friend🤷
Why is my age bothering him?
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