I think 15 is marginal. There has been some recent discussion over here as to what is the minimum legal age to leave a child alone in the house. Apparently, the short answer is that there is no legally defined minimum age, and it is down to the discretion of the authorities if they think the child is too young. Unsurprisingly, many people think this is an unsatisfactory situation.
If you are talking about regularly leaving your teenager alone for extended periods, I would think that 16 is a more realistic minimum age. Its a long time ago now since our children were teenagers, but I think we first went away on holiday and left our son in the house on his own when he was 16. We left him with a pile of food, a bit of money, and instructions not to burn the house down!