Spot on, but I think 'perhaps the problem' is being rather kind. To me, you, (and obviously others, as evidenced on this thread) it is the crux of the problem, which is twofold; firstly the lack of a rule leads to basic confusion as to what the qualifying date for age groups is, unless it is - in each case - specifically stated for each race / on each entry form. Secondly, it leads to the obvious problem different race qualifying dates would give if they were part of a within-season championship series, should the qualifying date (however defined) fall within that championship season. For a junior race in isolation it might not matter too much, but once it's part of a series its of vital significance, as many of you have pointed out.
I think several of you / us are repeating ourselves and going round in circles / banging heads against the wall, but WP's example of the UKA XC rule format does indeed ram the point home. It would be nice to think there could be a simple unambiguous one sentence rule, but clearly things like this (dates that impact seasonal series of grouped races) just have to be spelt out to the letter, and if that needs several paragraphs to avoid any confusion (like XC Rule 507), then so it should. I suppose you could say that page 108 of the FRA calendar does a similar thing for the junior champ series, but it leaves the rest of the multitude of junior races out on their own. If that's intentional fine, but I suspect its not, and that its just a side effect of not having a FRA rule.
PS (a breakfast muesli moment):
Of course what hasn’t changed, indeed, is that prior to 2013 there was no FRA age group rule and there still is no such rule in 2013. But at least before 2013, there was a handy convention in the ‘industry’. Which was, as the ex-FRA-JC and others point out above, that if in doubt (unless otherwise specified) assume the same date-basis as the Junior Champs, which happily and handily was aligned with the maximum distance rule date-basis. In fact this handy alignment / convention was so handy, it seems, that many misinterpreted it as a de-facto FRA rule. With the 2013 change to the maximum distance rule date basis (for perceived good reason relating to the Senior transition opportunity), the lack of a Junior age group rule now means confusion regarding any useful convention. This is manifest in the opposing views from the ex-FRA-JC and the ex-FRA-Chair as to what the 2013 convention should be, enhanced by an ambiguous calendar (in places, i.e. some races) and an ambiguous layout of the default FRA J- entry form.