It is correct. The adjective always end with -e when one uses 'der', 'die' or 'das'. This is the rule:
The red wine. The little girl. The young woman.
"Guter Junge" is correct, but only without the definite article 'the' or with the indefinite article 'ein': "ein guter Junge".
Hope this helps. I don't understand why this was written in German when we are trying to learn German!!
This is only true for the nominative case and only for singular nouns - adjectives for plurals take the -en ending.
I had a lot of problems trying to figure out how adjectives are inflected by following partial rules like this. Take a look at some of the online references where the rules are explained more fully. I often go back to the wikipedia page:
http://www.nthuleen.com/teach/grammar/adjektivendungenexpl.htm The explanations and flow chart on the above website are really clear and extremely helpful. Hope you get a chance to look at it.
Only strong adjective declension takes on the 'er' ending.
Weak adjective declension (with a definite article) either takes on either a '-e' or '-en' ending depending on the case and the gender.
Here are some tables showing the strong, weak and mixed adjective declension: http://en.wikipedia.org/wiki/German_declension#Attributive_adjectives