Some important definitions:
Adjective: this is a word that describes a noun.
Compound Adjective: when 2 or more words are grouped together and make one adjective.
Some compound adjectives are hyphenated, and some are not. Whether or not you should use a hyphen with a compound adjective really depends on where it is placed in the sentence.
YES - use a hyphen
It's good practice to use a hyphen when a compound adjective is placed before a noun.
- It was a communication-related matter.
- The team's long-range plan is looking good.
NO - do not use a hyphen
Do not use a hyphen when the compound adjective is placed after the noun.
- The matter was communication related.
- The team's plan is long range.
Using hyphens with age
The same rules apply with respect to the placement of the compound adjective.
YES - Saku is a four-year-old cat
NO - Saku is four years old.
However - When the compound adjective describing age is used as a noun, please hyphenate.
Example: I have two cats, Saku and Alfie. Saku is a four-year-old, and Alfie is a three-year-old.