There was a good deal of experimentation starting in WWI, when metal turned out not to work very well. I think the biggest obstacle is that kevlar is so counterintuitive - really thin filaments woven into really pliable fabric stop bullets?
Now, if anybody had done serious research into Chinese 'silk' and 'paper' armour (that's a simplistic descriptor but I don't know what they're called in Chinese), there is a good chance they would have come up with the principle earlier. Early variants would have been bulky, ridiculously expensive and very uncomfortable, but probably a bit better and a lot lighter than the WWII 'flak jacket' with its monster inserts. I'm not sure any synthetic fibre before '45 was good enough for the purpose, so these things would eat silk and a lot of people at DuPont would spend the war breeding spiders. If Nylon was already good enough, ballistic nylon is also a likely early development. From then on, the industry will just try to use any synthetic fabreic or foil to see if it works, so kevlar will be an early candidate and have its stellar career.
No big difference on the battefield, I guess. Armour is nice, but it doesn't actually stop bullets from hurting you.