Pictures of a Beckham working out would normally be cause for celebration, but when 12-year-old Cruz (the third child of David and Victoria) posted a clip of himself at the gym on Instagram this week, there were disapproving tuts from a certain sort of internet user.

“It’s actually bad to work out when you’re a kid; your muscles get damaged,” warned one, below the video of Cruz on a rowing machine, viewed almost 200,000 times. “You are too young for this,” wrote another. A third suggested he “eat a donut and have a childhood”.

But what’s more dangerous: the rowing machine or the donuts? British children are some of the world’s least active, with a recent study finding that just 15% of girls and 22% of boys do the recommended hour of moderate physical exercise per day.

According to Chris Smith, a spokesman for the Chartered Society of Physiotherapy, Cruz is more of role model than a cautionary tale. “Given the obesity epidemic, I think it’s really positive that a 12-year-old boy is taking an active interest in a healthy cardiovascular lifestyle,” he says. It’s fine for a young teenager to work out, Smith adds, so long as they seek advice from a specialist before they start weight training.

Vikki Anderson and her husband Terry manage the CrossFit All Out gym in County Durham, where they offer fitness classes to children as young as three. However, they only introduce weights at the “CrossFit Academy” level, for those aged 11 to 16.

“The coaches are extremely aware of making sure the kids are capable of doing what they’re doing, and doing it safely,” says Anderson. “Using their bodies to gain strength can be really empowering. It can lead to a love of all kinds of sport. It’s not just about going to the gym.”