I don't think you can say they're missing out on Youths football really as they are still getting the same years playing underage as everyone over the past few decades.
I do agree with you that players who are 18 when the league kicks off should be allowed played U18 football and likewise for other age groups but January has always been the month they go on.
When I heard they were introducing an U19 age group this season I thought initially it was a fantastic idea as it gives young lads an extra year underage before Adult football but when I heard that they have 'new' age group rules and read through them I thought to myself ''what is the point in this????". U19 should be for players born in 1992 and after not '93.