i started a few weeks before i turned 18. side effects i had was drop in libido - only a v small one though, and only lasted about a week. it didnt suddenly jump up and down (the libido, that is!) but went up constantly till it was what it was before. after starting finasteride ive never looked back. personally, although this is just my opinion, i reckon finasteride would be easier for people in their late teens/early twenties to handle in terms of side effects - younger peoples bodies are most used to big hormonal changes, after all theyre just finished/finishing puberty.