I am 20 and broke my 5th metacarpal. i hear people saying their breaks ache after being healed for years. is this guaranteed or only in some cases?

Varies : It's not guaranteed to hurt years later and depends on the severity of fracture, whether the joints were involved, open fracture, high impact trauma, medical health, nutrition status, delay of treatment etc. At your age, you should recovery fine if properly and promptly treated. Best of luck.

Not always the: Same for all people so i would not guarantee anyone that they'll have pain for years after a fracture in fact its typically the opposite in a healthy person in their 20s, usually once it heals discomfort subsides quickly. In severe injuries pain or the vague discomfort related to weather or cold can continue for a season or two but not for a typical minor fracture.