If you're worried about the safety of Penatropin (for any age group, not just teens) then the answer lies in the side effects of teh active ingredient.
I looked up what exactly makes Penatropin work, and apparently it is a "medicinal herb" called Butea Supera.
Is Penatropin and Butea Supera safe to use?
After some research on a few medical sites, it seems the overall answer is that not enough research has been done into Butea Supera (Penatropins active ingredient) to determine whether it is safe or not.
That is why it is not available in pharmacies, but can be purchased online where medication is less regulated.
So considering that, I would say "don't risk it for the time being".
Not just because of side-effects, but also because, if no official research has been done into its safety - then this also means no FDA approved research has been done into its effectiveness either.
All you have to go by is word of mouth stories about how this stuff has been used in far away countries such as Vietnam, the Philippines etc..
This is what one website had to say:
Butea superba is a vine that grows in India, China, Vietnam, and Thailand. The roots are used as medicine.
"People take Butea superba for sexual performance problems (erectile dysfunction, ED) and lack of interest in sexual activity. They also take it for diarrhea, painful or difficult urination, and fever.
It is not known how Butea superba might work as a medicine. Some evidence suggests that the chemicals in Butea superba may act similarly to hormones that regulate sexual function."