Some start at 3 years old. Some starts in their 50s and 60s. Usually a dojo (training place) has their rules on age as well. My dojo in Japan used to tell us to start at the age of 7.

Can I start kendo at 15?

Kendo is not a sport, but rather Budo, so that's why respecting the etiquette in Kendo is very important.

How hard is it to learn kendo?

If you mean it is difficult to learn by tough, it is always hard to learn something at the beginning. But you're only 19, so kendo won't be that hard to learn. If you want to be good at kendo, of course you have to train very hard. In that sense, yes, kendo can be very tough.