A lot of surveys can have huge biases in age since it comes down to who has free time (or if done near a university...), although with phone surveys it can be a strong bias to seniors. There would probably be a big bias toward younger people with free time on the net, while conventions might do better or even be biased toward older people on the basis of older people being more likely to have money to travel across country or further to a convention.
There are things that can be done to fix age bias, but I only know of things that require prior knowledge of the age distribution.
A lot of surveys can have huge biases in age since it comes down to who has free time (or if done near a university...), although with phone surveys it can be a strong bias to seniors. There would probably be a big bias toward younger people with free time on the net, while conventions might do better or even be biased toward older people on the basis of older people being more likely to have money to travel across country or further to a convention.
There are things that can be done to fix age bias, but I only know of things that require prior knowledge of the age distribution.