To start a dweller at level 1, they normally will be a child as some from the wastelands already have some level of experience.
To get a child, leave a male and female adult in the barracks while you have room for more dwellers, eventually the female will get pregnant (if they aren't related, swap people around if you're having trouble). After awhile the pregnant dweller will have a child, who'll be useless for awhile before 'growing' into a adult dweller - then level them up to 50! :)