How old was Cheung 3 Fung when he founded the Mo Dong Sect?

The founders of wulin sects are typically at least forty years old when they found their sects. After all, they need time not only to acquire the martial arts skills to make themselves credible as martial arts teachers, but they also must accumulate enough wulin experience, wisdom, and resources to start and support the sect.

How old do you believe Cheung 3 Fung was when he founded the Mo Dong Sect? I think he might have started even later than others (maybe not until he was at least sixty years old). At the beginning of HSDS, Cheung 3 Fung celebrated his 90th birthday, but his most senior disciple, Sung Yeun Kiu, was only in his thirties. Unless Cheung 3 Fung had another generation of students prior to the Mo Dong 7 Heroes of HSDS (which isn't likely), then Cheung 3 Fung didn't found the sect too much earlier than thirty years before the HSDS story started (e.g. when Cheung 3 Fung was around sixty years old).

I think it was mentioned that he was in his fifties when he came down from seclusion in the mountains.

Cheung Gwun Bo (later Cheung 3 Fung) was born three years into Little Dragon Girl's sixteen-year separation from Yeung Gor. Hence, he was thirteen when ROCH ended and sixteen when Gwok Seung visited Shaolin Temple three years later.

At the beginning of HSDS proper, Cheung 3 Fung is ninety years old. However, his oldest student, Sung Yeun Kiu, was only in his early to mid-thirties at the time. The other six Mo Dong Heroes were even younger, and the youngest, Mok Sing Guk, was probably in his early twenties or even late teens (eighteen or nineteen, though more likely twenty or twenty-one).

Think about what this means: the Mo Dong 7 Heroes were the first seven Mo Dong Sect students. It's not like there was a Mo Dong Sect before Cheung 3 Fung recruited those seven young men, but if the oldest of them was only in his thirties at the beginning of HSDS, then the Mo Dong Sect could not reasonably have existed more than fifteen or twenty years before the beginning of HSDS (it's not like Cheung 3 Fung recruited and started training Sung Yeun Kiu the moment that Sung was born, and his younger sect brothers would have naturally come along even later). That means that as of the beginning of HSDS (Cheung 3 Fung's ninetieth birthday), the Mo Dong Sect was a relatively new development...something that Cheung 3 Fung started a mere fifteen to twenty years earlier when he was in his seventies! Given how well-established that Mo Dong Sect seemed at the beginning of HSDS, with a huge reputation and large base on Mt. Mo Dong, it seemed like something that Cheung 3 Fung would have established forty or fifty years before the beginning of HSDS, when he was in his forties or fifties (generally the age range that people such as Central Divinity Wong Chung Yeung and Gwok Seung established their sects), giving the Mo Dong Sect enough time to establish the reputation and veneration it had at the beginning of HSDS. That, however, wouldn't jibe with how relatively young the Mo Dong 7 Heroes were at the beginning of the story.

Curious. Most curious.

The most like likely scenario to me is that he started picking up SYQ, YLZ etc. first, raised and trained them in martial arts to certain age then thought about founding Wu Dang. However, it wouldn't be until SYQ, YLZ started roaming jianghu to perform their good deeds that people began to know Wu Dang. Before he founded WD, technically, he was still a Shao Lin's traitor so I don't think he wanted people to know about his new sect until its reputation has been established organically.