Could be water or antifreeze getting into the oil
Just save your money and look for a decent used bike for a first bike. There are lots of good used dirt bikes if you know what to look at when buying a bike.
dirt bike i would look for a 200-300 cc street i would look for a 650-700
look for leaks and craks and dents
look on the front on the frame you will see a vin numberwrite that down and call a dirt bike mechanic or call suzuki
Google "how to replace dirt bike tires" or look at forums at thumper talk http://www.thumpertalk.com/forum/showthread.php?t=390735
I would look for a nice used yz125 to start out with. There are a lot of them and they were top of the class for a long time. The crf150 answer was stupid.
I would look at a Yamaha tt-r225 which would be comparable to a crf150r in power. The new Honda bikes are garbage, don't waste your money.
Justin Bieber's bike is a dirt bike. On the dirt bike it has a picture of him and his ex girlfriend. The dirt bike cost $10,000 which is a lot of money. The speed of this bike is 9 miles for 30 minutes.
Those are just some dirt bikers, you can also look under "dirt bike dictionary" for more info on models, of dirt bikes and so on.
Sites I recommend to look at: Craigslist.com or other sites common to craigslist.com in your area.
It looked like a regular bycycle, but with a motor and just a little widthier than a bike.
A 110cc dirk bike is a fairly small and short bike. It would probably fit an older child of 10 or so, but be too big for a 15 year old. You may want to look into bigger bikes for a teenager.