What's the right punishment for a case like this? Does anyone believe that full rehabilitation is possible, and that a teenager who does this will later come to realize the error of their ways and become be a valuable contributor to society sufficient to make up for this? Or is the sole goal a punishment that will deter others from doing similar anti-social deeds?
Remove each month some amount of money from his bank, so somebody is hired to plant, watch and care for some trees in an unknown protected location. Remove resources from the offenders and nurture the healers with this resources.
Repeat this procedure for the next 50 years of his life until the idea that boycotting nature for fun is not funny at all gets firmly established in their brain and he understands how much economical value has an old tree. Stop the chain of "crime school" challenges so we can protect children also. Use it as a clear warning for future criminals.
Eye-balling this, the 90th percentile 16 year old is about as tall as the 80th percentile 20 year old, and as heavy as the 75th percentile 20 year old.
Conversely, the 50th percentile 20 year old corresponds to like the ~55th percentile 16 year old in height and like the 75th percentile 16 year old in weight.
While true, this isn't the right way to analyze this. The right question is how much strength or size is necessary to operate a modern powerful chainsaw with a sharp chain. The answer is "not much". You need to be able to position a ~15 lb (7 kg) saw at waist height for a few seconds until each of three cuts is started. With a sharp saw, the forces are negligible after that for 2 of the 3 cuts required. With good technique, you can even use leverage against the trunk for the one short upward cut. There is considerable skill involved, but brute strength is not a requirement. While you are presuming a male 16-year-old here, I wouldn't be surprised if most female 14-year-olds were physically strong enough.