18 is a Medium and you are a Large. And it's an old MTB so it sits a lot more racy than a new one. A 1990's medium would feel tiiiiiny after coming off a 2023 large bike.
Just for fun...
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Genesis 49:16-17
"Well, well!" said Holmes, impatiently. "A good cyclist does not need a high road. The moor is intersected with paths and the moon is at the full."