**Core Concept**
The question is testing the association between early age of sex and various health risks. Early initiation of sexual activity has been linked to an increased risk of several health outcomes, including sexually transmitted infections (STIs), unintended pregnancies, and psychosocial problems.

**Why the Correct Answer is Right**
Early initiation of sexual activity can lead to a higher risk of STIs due to the lack of experience, knowledge, and protective measures. This is because younger individuals may be more likely to engage in high-risk behaviors, such as having multiple partners or not using condoms consistently. Additionally, early sex can disrupt emotional and psychological development, leading to issues like depression, anxiety, and low self-esteem.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not supported by evidence and is a distractor. Early age of sex is not directly linked to an increased risk of diabetes mellitus.
* **Option B:** While early age of sex may be related to an increased risk of mental health problems, this is not the most directly related option. The correct answer is more specific and evidence-based.
* **Option C:** Early age of sex is not a well-established risk factor for cardiovascular disease. This option is a distractor and not supported by evidence.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that early initiation of sexual activity can have long-term consequences on physical and mental health. Healthcare providers should counsel adolescents and young adults about the risks and benefits of sex, providing them with accurate information and resources to make informed decisions.

**Correct Answer:** D.