Umai is a Japanese word that refers to delicious or tasty in English. “Umai” is not just used by young people, but all the Japanese people say this word naturally when they eat delicious food or drink. There is similar word, “oishii” in Japan but most people use “umai”. The reasons might be that “umai” is much easier word to say than “oishii”, and it sounds more cool to Japanese people.
Also, there is other way to use Umai. A person who does beautiful thing or to the great sport player, we says them “Umai person”.
As for me, I don’t say “umai”. I say “Uma!” because it sounds more cool. Nowadays, some of the gourmet reporters say “maiuu”. I hate to hear this word because it sounds so bad. Umai is very nice word but in some case this word gives a bad impression to others because it is known as a lower word than “oishii”. So sometimes you should use “oishii” instead of “umai”.