My memory has stayed the same or even improved over the last year. does this mean i don't have dementia? i'm 21 scored 25 on moca

Probably no dementia: Without knowing much more about you - I can't make a firm prediction - but from what you say - you have no likelihood of having dementia. If you know of an injury or a substance that did some cognitive damage, it's still not exactly dementia and your score is borderline fine. Some very mild impairment may be experienced, but you can target it w/ therapy. Relax!