Is melatonin safe for a 16 year old to take for insomnia? and if so are there any side affects?

Relatively safe: There are very few studies using melatonin in children and adolescents, but they lean towards safety: http://www.Ncbi.Nlm.Nih.Gov/pubmed/12635982 the most commonly reported side effects of melatonin are headaches, dizziness, nausea, and drowsiness. Doses from .5mg to 10mg have been used, but starting on the low end might be wise -- along with help from a parent and/or your doctor.

Safe: You should take it 2-3 hours before the target bed time, the side effects usually sleepiness in the morning at high dose, therefore start at 0.5-1 mg first.