I don't know what the standard of age group county cricket or how you would go about getting picked.
What I can tell you relates to hockey, getting to regional level as a junior requires you to be in the right program and you have to be nominated to JRPC trials at the various age groups, if you make it through JRPC trials you get to go to HiPAC u15/u17 or the Futures Cup u16/u18 and from there to the National Age Group squads.
I'm guessing cricket will have a similar set up and if you haven't been nominated I would guess that either means you are not considered good enough or the person who is supposed to communicate this stuff hasn't done it.
If getting to into a county age group is your dream then by all means persue it, by the sounds of it you may need to move to a club with an established history of getting juniors into the county set up, if you do move to one of these clubs then make sure you listen to the coaches, if they tell you that you need to work on something to get noticed then go away and work on it.