When I say I have one brother and he is 27 years old, do I say
J'ai un le fr¨¨re or J'ai un fr¨¨re
Il a vingt sept ans.
The definite articles meaning "the" are le, la, l' and les. The INdefinite articles, meaning "a, an, one" are un, une and the plural "some" = des.
When you say J'ai un le frère it will mean "I have one, the brother." Since your English says "and" here's the sentence:
J'ai un frère et il a vingt-sept ans.
This is a good review for you of both the definite & indefinite articles!
P.S. Let me know how your text is set up, since I'm not at all familiar with it. For each lesson, are there "explanations" for the grammar you are covering? If you let me know, lesson by lesson, the grammar points you are covering, I can post "explanations" for them, if you'd like. Each year we always begin with a review of French I where you get a lot of basics, French II, where there are most all of the rest of the basics, French III, with the "fine points" of Subjunctive, etc. and the funny thing is that in French IV/V nearly all the errors the students made came from French I or II!
Mme
When you say "I have one brother and he is 27 years old" in French, you can say "J'ai un frère" (I have a brother) or "J'ai un frère qui a vingt-sept ans" (I have a brother who is 27 years old).
In French, it is not necessary to use the article "le" before the noun "frère" (brother) unless you want to specify a specific brother. In this case, since you mention that you have one brother, it is clear that you are referring to him, so you can omit the article.
To state your brother's age, you can follow the sentence with "Il a vingt-sept ans" (He is 27 years old), as you mentioned correctly.