To qualify for baptism you first must be a publisher. I think I read once that 25 % of witnesses were not yet baptized.
A comparison between the increase in publishers with that of baptisms is not accurate because the peak and average publisher figures already include those that were baptized that year. For instance, the 2010 YB reports a peak of 7,313,173 publishers for the 2009 service year. Of those, 276,233 got baptized during that service year. Those newly baptized ones are already counted in the publishers figure as you cannot be baptized unless you have already been a regular publisher reporting your field service for at least 6 consecutuive months prior to your baptism. I became a publisher in 1976 but did not get baptized until 1978. So I was already reporting field service each month and was counted as a publisher in the peak publisher figure for the 1977 SY and from 1978 on.
I think if someone can give a presentation at the door by themselves, they are old enough to become publishers. Age may vary, it's up to the service commitee but I knew kids as young as 9 and ten reporting as publishers.
I hope this helps answer your question.