Learn English – Difference between “so rude” and “too rude”

Fast food is served at a fast food restaurant, such as Mcdonalds, &c; while junk food a specific type of food such as chips, pop, &c.

Obviously, junk food could be served as fast food.

Essentially, the adjective: "fast" modifies the noun "food" in describing how long it takes the consumer to get the food ready to be eaten, that is the food will be ready very soon.

The adjective: "junk" describes the quality and nutritional value of the food. Here "junk" is considered as that which is not usable, i.e. garbage.

Therefore to answer your question, different aspects of the food are being described.

1. preparation time
2. quality

They are not quite the same, but they are similar.

Two-year-old is an adjective. You can say, two-year-old girl, or two-year-old cat, or two-year-old child. Sometimes, two-year-old is used as a noun on its own, and it that case ("My two-year-old", say), child is usually implied, although it could refer to an animal if the context is clear. It's a compound adjective, with dashes to make it clear it is all one phrase, and it is usually pronounced with the words run together. You might see it without the dashes, and it is usually clear what is meant.

You can't say "My daughter is two-year-old". In that case you say "My daughter is two years old".

I think the reason why it's year, when with dashes, is because the words are run together, and years would be hard to say.