Also is there any oral minoxidil, or is that just a stupid question?
Minoxidil (active ingredient) was initially developed as an oral agent (Loniten), to reduce blood pressure in guys with heart problems. It acts as a vasodilator, and hair growth was noted as a side effect of its use -- hence they made it into topical minoxidil.
You definitely do NOT want to take this drug orally. Even some guys that use it topically report increased heart rates, heart palpitations, dizziness etc.
if i take finasteride before 18 then the side effects will just be more intense.
It's not necesarilly about side effects since you won't know if you get them until you take the drug -- but the side effects are a direct result of the drug's many mechanisms of action, the main one being reducing systemic DHT levels ~ 70%. Both 5AR-II AND DHT have a role to play in your body besides making you lose hair (ie, 5AR is involved in neurosteroid synthesis in the brain), and blocking them with finasteride at your age may have extra unforseen consequences since your body is still develpoping physically and hormonally.
Will the side affects be more likely or is there the same chance of gettin side affects but the affects will just be more intense
Merck claims only 2% of men get side effects (as noted in their FDA Propecia Trial), but many consider that number BS as various other studies claim much higher side effects percentages. Obviously Merck wants to downplay the potential problems with this drug since they are trying to market it in the best possible light, so it's up to you as a consumer to evaluate non-biased info before you make your decision.
All I can say is this: the drug is NOT meant for teenagers, there is no guarantee you will have side effects, and if you do, that they will go away when you quit -- for most men they resolve within 2 weeks/month after stopping, but for a smaller unknown percentage, they persist... and often, these guys have a major drop in Testosterone levels and complete loss of sex drive after quitting... and nobody knows why. The drug can really do a number on your endorcrine system -- it is essentially a feminizing drug and acts as a chemical castrator for your Prostate.
If the chance -- however slim -- of losing your sex drive (never horny), not being able to get it up, growing tits, having almost no jizz, and penile shrinkage and fibrosis (rare) and lots more bad stuff appeals to you... well, there's not much anyone can do to stop you from taking it. But if you value your health and sex life AT ALL, you will not risk it at this stage.