There are so many factors that cant be account for with age, maturity and experience. I beleive a mature 18yr old with a liter bike (while he DOESNT need it) woudl be fine with it. But at the same time I had a buddy get into bikes b/c of me, he bought a 99 zx6 I beleive? I dunno, but it was an old Kawa 600 that was a TANK of a bike. He rode it for a monthor , went and watched me do a trackday and said he was selling it to get a GSXR1000. I told him he shouldnt, as did all the guys at the trackday.
I told him he should get a 2006+ GSXR600, since that bike is leaps and bounds better than his old carb heavy tank 600 and he would notice the power different and weight difference immediately. So he wouldnt listen but i finally got him to decide on getting a 750 instead of a 1000. Now I worked with this guy and I know he has a tendancy to have to be the bad ass, and have the best stuff and always has a story better than yours (one of those guys)
He tells me he's going to buy his bike one day and I said dont get a 1000, my boss which is a harley rider says "dont be a b!tch get a 1000" I reminded him of all the 1000's he watched me pass on the track in the straights b/c I've ridden 600's for 8 years and I still havent tapped into all the power my R6 has. He shrugged me off. He came into work the next day on a brand new GSXR1000. I rolled my eyes and didnt pay attentino to him, also he wanted a Gixxer b/c everyone else rides them.
No more than a week later he's pulling out of the dealership with our other friend, that listened to me and bought a gsxr600. He turns out of the dealer, spins his back tire and highsides his bike. Luckly unhurt.
He fixes the damage, him and our friend ride home to Texas for holiday, he wrecks again trying to pop wheelies and flipped the bike over.
At this point our friend said Im not riding with you anymore with that bike b/c evidently he's wreckless everywhere they ride. He fixes his bike again, wasnt a month later he goes home again and showing off for his friends pops another wheelies and flips it again, this time really hurting himself. The bike is fixed now, he's still hurt, but how many times it it gonna take?
Our friend has been riding for over 6 months now, I rode with him a couple times and he's very cautious and a good rider. He wants to learn without an ego. He wanted a new bike and he wanted a ZX10R b/c he liked the way they looked, not the power or status, but he honesly just wanted teh look of the bike. So he bought it and has been perfectly fine on it, he respects the bike and the power however which is a big factor.
IMHO age has nothign to do wiht it, its personaliy and maturity. I've had a 30+yr old on a honda 929 tell me there is no way i could beat him on a track (he's never been on a track) simple b/c he has a bigger bike. That ignorance is what gets people killed.