No, there wouldn't be. The unit side of the tech tree doesn't give any indication that some units are era-dependent. Cynwulf is also correct in that there was a bug in the unpatched version of vanilla MTW allowed you to train any of your faction's units as long as you had the requisite buildings, regardless of era. Thus, the English could train Longbows in the Early period, and the Byzantines could recruit Varangian Guards in the Late period.
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