Hi. Thanks for the question.  There isn't really a set age range where kids can train together.  In affiliated youth football matches there is a 2 year age banding so a player can play one age group above their own, until U18s where it is a 3 year age band.  The key things to consider are whether the kids can cope socially, physically, psychologically and technically.  Friendship groups are very important so this needs careful thought as well as the balance between challenging the kids that need to be stretched and helping the kids that find it more difficult.  The trick is to to provide opportunities for all that aren't too difficult and yet aren't to easy either.  Within the agree range you mention I would probably be looking at segmenting that group into 4-6 and 7-9, depending on numbers.  Don't be afraid to speak to your players and get their feedback on how it is working and how you are grouping the kids.  Let us know how it goes. 

While a historic post, I'll give my two cents because we are in the exact same position right now re the 4-9 age group. We have two training nights, so we have the 'reception' kids on one night and the rest on the other. The younger kids are actually more work (in a good way) meaning they need extra attention. They would be totally lost in a larger group of older kids. So this arrangement works very well.

We then have the 6-9 age group together on the 2nd training session. Now, this has been creating its own issues when it comes to the small-sided games. We have tried to spread ability evenly among the teams. And, while there is value in pitching kids in among their peers, what we are also seeing is some of the smaller and/or less talented kids having less involvement in the games. From next week, we are going to divide kids by year and pick the teams that way, e.g. Y1s, Y2, etc. Then, we will move kids up or down depending on how they perform. 

I guess there is no perfect solution but we will see how this goes.