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).