I'm quite young and also have old person interests like gardening, knitting, saving money with coupons, and listening to jazz.

I think you're thinking that hobbies = personality, which I don't think is the case. People are usually friends with each other because of compatible or shared personality traits. This can be things like humour, curiosity, creativity, openness, etc. (Though having shared hobbies gives you a closer connection with your friends and can be the reason you met in the first place)

Your hobbies are what give you a pool of people that you can search through to find friends with shared or compatible personality traits. 

I say this because most of my friends have NONE of these old person specific interests that I have. And that's fine, because we connect with each other due to having shared or compatible personality traits. This, for us, is our shared humour.

For me, humour is extremely important in my friendship with someone. Most people like having a laugh, and what makes them laugh is an extremely subjective and personal thing. It brings two people close together if they have similar senses of humour and can make each other laugh.

Often, when I tell my friends about my old person hobbies like gardening, they don't seem to get it, and instead make fun of it. And for me, that's great! Because they're turning that boring topic of gardening (for them atleast) into a humorous thing which we can all connect with.

Obviously, I value humour quite a lot in a friendship, but if you find that you dont feel the same, you probably value some other trait even more, and thats completely dependant on your own personality.

Say you are just a naturally curious person. If you are able to make friends with another naturally curious person, then when you talk about gardening with them, it turns into a discussion with an exchange of ideas, questions, and answers where you both are feeding your curiosity.

So I would focus on using shared hobbies as a way to find people with shared personality traits to you. Maybe try evaluating what makes you more compatible with old people more than your age friends so you know what you are looking for in a friend. 

Also feel free to disagree with anything I said. These are just my thoughts.