There is no definitive answer to this question as it largely depends on the individual's strength, size and level of athleticism. However, based on research and anecdotal evidence, it is generally agreed that the average squat for a 15 year old is between 135-185 pounds. This range will obviously differ based on the factors mentioned above, but should give you a good idea of what to expect. If your child is interested in competing in powerlifting or other similar sports, then they will likely need to increase their squat weight significantly. However, for general strength and fitness purposes, the average squat for a 15 year old should be within this range.