1. Biological age is the matter not the date when you were born. Assuming you are healty with your inner glands working correctly, yes, I think you are too young. You can still improve to reach your genetic limits which I presume you cannot get in just three years.

2. If you had good result so far, why changing? You are just 23, keep going like that. And 150 grams of protein per day are more than enough to grow naturally, it's 2.2gr of proteins per kg body weight.

3/5 I have no knowledge to answer.

You seem a clever young man, so I would suggest you to go and speak with an endocrinologist. Take a professional BMI exam and see what you really have accomplished so far, check out which parts of the body must be trained more to compensate the others, get in touch with your inner fats and least but not last, figure out your basal metabolism. With all those information (BMI), I am sure you can improve your diet correctly and keep being natural.

By the way, 8%BF? How did you get that result?