BEIJING - China's social media platform WeChat was abuzz with people posting photographs of themselves aged 18, on Saturday and Sunday. The trend showed many Chinese readying themselves for 2018 in nostalgic mood.

But why 18?

On Sunday, even the youngest members of China's post-90s generation -- those born on Dec 31, 1999 -- will turn 18 and reach adulthood in China.

For many, this milestone suggests a generational shift.

"Now the stage has been set for the post-00s generation," said an Internet user.

For many, posting the old photos was not just a coming-of-age party for the post-90s generation, but an occasion to lament the passing of time. "I was young and chubby. Now I am old and thin. I would rather exchange the lost weight for the lost time," said one wistful WeChat user.