I think that when a nine-year-old kid says something fits," what he/she really means is, I can climb inside." So I am not at all sure that an adult medium helmet fits, or anything close. And this matters. Bike helmets have saved my life once and saved me from serious injury twice, so I am an absolute hardcore believer in their efficacy.
You should do a fit test. Have the young man don said helmet, and attach the chin strap. The helmet should be level and snug around the head. And the chin strap should also be sung; he should be able to open his mouth to talk or eat, but not much else. Can you wiggle a finger up under the helmet rim? Can you easily pull the helmet off by pushing it forward or back? If the answer to any of those questions is yes," then the helmet does not fit. Period. To do any good, a helmet must be snug and level.
That said, at age nine, perhaps a small adult helmet will fit just fine. And Id certainly recommend him having his own helmet, rather than borrowing yours. You can fine-tune fit even with same-size helmets.
There are lots of possibilities. Giros Indicator ($40) is an excellent helmettested and approvedwith lots of vents and a cool-looking visor. Bells Trigger ($35) is sort of a tweenera youth helmet that should fit your son well. So go to a bike shop and try on some helmets. Let him get what he likes! That way, hell wear it.
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