Although clothes, architecture, etc. heavily imply a barely post-Roman (5th-6th centuries AD) setting, characters still use the terms England and Normandy, which aren't recorded before the 9th century. It would be more appropiate to refer to them respectively as Britain and Armorica or variations thereof.
Every actor playing Merlin at different life stages has a different eye color. Baby Merlin has dark brown eyes; teen Merlin has unnaturally bright blue ones; adult Merlin has normal blue; and elderly Merlin has brown again.