I sometimes hear Japanese say “他人の子は成長が早い.” Although there isn’t an equivalent phrase in English, you could translate it as “Other people's children grow up so fast.” and keep the same nuance and meaning.

It is true, isn’t it? Whenever I come across an old friend of my boys for the first time in years, I am always surprised by how much they have changed. Living with my own boys and seeing them day after day, however, it’s hard to notice the changes that are happening little by little. It’s only when I come across a photo from ten or more years ago that it hits me how big my sons have become. It’s almost as if an earthquake has happened. My older boy, for instance, is now taller and heavier than me. Only 14 years old but almost 178cm tall. Hard to believe that a little over a decade ago, he could comfortably fit in our bucket. (Hard to believe I still have that bucket, too.)

自分の子も成長が早いものです。