I get white spots under my nails, but I've had them since I was little?

Zinc deficiency? There can be many causes but the most common are injury, nail fungus and zinc deficiency. If you've had these since childhood i would suspect zinc deficiency might be the cause. See http://www.Md-health.Com/white-spots-on-nails.Html ; http://drlwilson.Com/articles/zinc.Htm.
Leukonychia. In medical science white spots in fingernails are also known as ‘leukonychia punctata’ – which related to the presence of nucleated keratinocytes (contrary: narrow white lines in the nails are known as ‘transverse leukonychia’). Usually white spots are caused by random minor trauma – which also explains why they are relatively common in the hands of children!