There is no character named **"Little Jacob"** in Grand Theft Auto IV. You are likely referring to **Jacob "J" Hughes** (often called **Little Jacob** by fans due to his stature/voice, but his in-game name is **Little Jacob** — *correction*: his name **is** Little Jacob).
Correction: The character **is** named **"Little Jacob"** (real name **Jacob Hughes**). He is a Jamaican arms dealer, friend of Badman, and associate of Niko Bellic.
Rockstar Games never states his exact age. Based on context:
He is **not a teenager or young adult**. The "Little" refers to his **short stature** (approx. 5'4"/163 cm) and possibly affectionate/diminutive nickname from Badman/Niko, not youth.