Well you won't tell me who all these mythical people who are going to be sent to jail over prop19 are, so I have to assume you're talking about the age restrictions being that this is the only conceivable way anyone could be sent to jail over prop 19.
Oh goodie... from the Initiative, from the added to Heath and Safety Code as section 11300:
(b) “Personal consumption” shall include but is not limited to possession and consumption, in any form, of cannabis in a residence or other non-public place, and shall include licensed premises open to the public authorized to permit on-premises consumption of cannabis by a local government pursuant to section 11301.
(c) “Personal consumption” shall not include, and nothing in this Act shall permit cannabis:
(i) possession for sale regardless of amount, except by a person who is licensed or permitted to do so under the terms of an ordinance adopted pursuant to section 11301;
(ii) consumption in public or in a public place;
(iii) consumption by the operator of any vehicle, boat or aircraft while it is being operated, or that impairs the operator;
(iv) smoking cannabis in any space while minors are present.
The bold are the others besides 18-20 crowd that you're so ready to cut and run on. I know in Golden Gate Park, it's not uncommon for people to gather on the weekend and have a smoke session. Same for Venice Beach in Los Angeles and Ocean Avenue in Santa Monica. Part (iv) is pretty self-explanatory on why it's not such a good idea.
Subsection (a) is the one we all know... blah blah over 21 can grow and cultivate for personal consumption. Gotcha, don't care about kids. They're old enough to work and pay taxes and go die, but not enjoy what they already have the right to enjoy. Not the way I do things, but OK.
Subsection (b) is a real tickler. While it seems to make ground-breaking allowances, we already have the right to consume in the home and non-public spaces. All it does make allowances for are retail centers. Adding more middle men doesn't do much to help the recreational user, in my opinion, but lots to help the middle men.
Subsection (c) I sort of already reviewed. Part (i) and (iii), are clever pieces of legislative trickery. "Intent to sell" and "driving under the influence" are already covered under existing Health and Safety Codes. By including them in this proposition, it makes it easier for law enforcement and/or regulatory agencies to set new guidelines for both charges, which we know are the most common charges utilized for nabbing smokers today. And with the taxes that are suddenly being funneled into regulation, that means more cops hired to bust smokers any way they can.