It's a
gogatsu-ningyo ("fifth month doll"), a Boy's Day armour. Since shortly post-WW2, it's Children's Day, 5th May. Earlier, it was just Boy's Day, and 5/5 on the lunar calendar.
The oldest I've seen are late 19th century, and the newest I've seen are brand new. Unless it's marked, all you have to judge the age is the condition (or maybe types of plastics and other materials used).
A kabuto/helmet is more common than a whole armour (due to cost).
Some info and examples:
Boy's Day:
http://www.nippon.com/en/features/jg00052/
Antique:
http://web.prm.ox.ac.uk/weapons/inde...rmour-asia-75/
Antique:
http://www.asianartbykyoko.com/items...-Kabuto-Helmet
New:
http://global.rakuten.com/en/store/k...o/item/13m-83/