There’s a mod that does something like this – see the thread here:
I haven’t played this mod and the description in the thread is not very clear – I think it originally had just an extra 5th age, but now 7 ages in total, with essentially no regard for historicity (e.g Bronze Age between Dark and Feudal Age). It seems to be roughly what the OP is suggesting, and if not, it at least demonstrates that something like this is possible with a mod.
If you care about ages representing historical periods, another option would be to split the Imperial Age in two. Since trebuchets don’t become available until Imperial Age, it ends up representing something like a 500 year period (whereas Castle Age is probably only about a 200 year period). Splitting it in two, with Imperial Age essentially as it is now but without gunpowder, and a fifth Gunpowder Age in which, say, castles become easier to take down and non-gunpowder siege weapons become obsolete, could make some historical sense without really stretching the timeframe.
On the other hand, it’s too radical a change to happen officially. It would have to be a mod, and some civs (e.g. those that currently get gunpowder early) would have to be substantially reworked.