The approximate unit would most likely be years. When measuring the age of a person, one normally uses years. That's why all our birthdays are only once a year. In addition, it probably wouldn't be a good idea to say something like months, days, weeks, or centuries because those units are too small or too big. Decades could be used in this scenario, but it's unlikely. You normally don't say you are 1.5 decades old. You say you are 15 years old.
