and how old do we gotta be to become a worker at dollar tree

At least 16 year's old.. You can only work part time due to the State law..At the age of 18 year's old you can be employed full-time..Each location is different about employment.. Contact the location you seek employment with.

18 in 99% of the stores

Not sure..... call the store location you want to work at, ask for the manager, and ask him all the employment-related questions you have.

Old enough to get a job

Don't know