It's not a common phrase, but the word "threshold" basically means the limit or boundary, so "age threshold" can mean either the highest age or the lowest age, depending on context. In this context, it means the lowest age. Currently, I believe you have to be at least 30 years old to get a Covid vaccine in Hong Kong. If 30 is the lowest age allowed, then the "age threshold" for getting the vaccine is 30. They want to lower the age threshold so that younger people can get the vaccine.

It means they are lowering the age requirement to 16 years old.