**Core Concept**
The diagnosis of allergic rhinitis in children is challenging due to the limited understanding of their immune system and the absence of clear symptoms. Allergic rhinitis is a type I hypersensitivity reaction mediated by IgE antibodies, which triggers the release of histamine and other chemical mediators from mast cells and basophils.

**Why the Correct Answer is Right**
The correct answer is based on the fact that the diagnosis of allergic rhinitis in children is often delayed due to the similarity of symptoms with upper respiratory tract infections. However, by the age of 5-7 years, children's immune systems mature, and their IgE levels stabilize, making it easier to diagnose allergic rhinitis. This is because the immune system's ability to produce IgE antibodies is more pronounced after the age of 5.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too early to diagnose allergic rhinitis, as the immune system of children under 2 years is still developing.
**Option B:** This option is also incorrect because the immune system of children between 2-5 years is still maturing, and it's difficult to diagnose allergic rhinitis accurately.
**Option C:** This option is incorrect because while the immune system of children over 7 years is more mature, it's not a definitive age to diagnose allergic rhinitis, as symptoms can vary.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the diagnosis of allergic rhinitis should be based on a combination of clinical symptoms, IgE levels, and skin prick tests, rather than relying on a specific age.

**Correct Answer:** D.