Depends on a number of things.
Ds was interested in business from age 4 or 5. At 12-13, I gave him a choice as to how he thought it would be wise for us to use the money for a cell phone: "We can do 'X' amount every 6 months or a year. Or sacrifice 'Y' . . . Or, you can pick out a cell phone for Xmas or birthday." He continued to leave me written notes, planned well, or last resort if he was going to be late, borrowed someone's phone (or he could have used the school's phone). I was a single mom.