It's an odd thing to marvel at how quickly celebrity kids grow up right before our eyes, isn't it? One minute, they're adorable tykes with oh-so-quirky names, the next they're Instagramming shirtless pictures of themselves. And you're left wondering why time is such a relentless beast.See: David and Victoria Beckham 's 16-year-old son, Brooklyn Beckham . The eldest of the genetically #blessed Beckham brood has already started making waves in the modeling world But the chip off the old beautiful block (he's pictured here with his mum) upped the whoa-wait-when-did-that-happen ante when he posted a photo on Instagram this week with the straight-and-to-the-point caption, "Working out."The photo, which is garnering a lot of attention around the web, will probably make you feel bad about your own workout routine. But mostly, it'll make you feel really, really old. Weren't you born yesterday, young man?!See for yourself.