The common thought is 5000IU per day is adequate for most. There is some debate as to whether synthetic D can actually be used by the body, some people do have bad reactions to it. A sun lamp or tanning beds are alternatives. Dr. Holick is the leading expert on D and his talks are great, he says D2 has shown to be just as effective as D3 by the way. https://www.youtube.com/watch?v=hiGBVDcbFVk
Another great source for info. https://www.vitamindcouncil.org/
What about for teenagers?
Which brand of vitamin D would you recommend?
I take Nordic Naturals Vegan Vitamin D3. It's made with the method developed by Vitashine, and I take 1000 IU per day. I get it on Amazon.