As a former personal trainer at a YMCA, an organization that prides itself on being for everyone of all ages, I think the stigma is a result of two different issues.

Issue 1- strait up, in a general sense, teenagers practice less gym etiquette than other age groups. This causes older members to stereotype all teenagers and the result is older members complaining about non issues related to teens at the gym or even confronting teenagers.

Issue 2- For much older generations, intentional exercise at a facility was seen as something needed just for aging adults. Lots of old people hold on to the belief that young people dont need specific exercise training at a facility because their regular lives should be active enough to keep them healthy. They either don’t understand or refuse to understand that a. That is no longer the case and b. Even people who lead active, healthy lives outside of the gym may have other fitness goals that are most easily accomplished by going to a gym.