Hi Martin,
As Luc says, dating menpo is very difficult - and that's coming from someone with loads of experience.
With reference to your menpo, it's hard to say from that one picture. I can give you a couple of guidelines, however remember there are always exceptions to any rule with Japanese armour. Older menpo (i.e. pre-Edo) usually have larger orifices, including larger mouths and ase-nagashi-no-ana (sweat hole under the chin). Also, older masks tend to be deeper. When it comes to Etchu-bo or yasuri men (those with parallel etched lines) from the Iwai/Yozaemon school, earlier examples have the nosepiece riveted on while later examples have a removable nosepiece.
John