Yang Yang was still nursing Fu Long at the age of about 2 years, until she didn't produce any milk any more. Fu Long was weaned naturally by his mommie not by the keepers! Fu Long was seperated from his mum at the age of 27 months, on the day, when he was transferred to China.
I don't like the idea to put panda cubs into a "kindergarden" at the young age of 4-6 months. But, in the kindergarden the pandas are allowed to join each other, they are living in groups of 4-8 panda cubs.
IMHO it's unnatural to wean = seperate cubs at the age of 18 months and force them to live alone, although they hear and smell mommie next door. That's what zoos in the USA are doing, just to get the next cub as soon as possible. This procedure is a bad experience for both mum and cub.