From what I've seen, I think the age/wait till your older deal stems from the vast majority of 18-20yo jumping on here after talking to their boy from high school who told them they got swole on x substance. With little knowledge of training/eating they want to throw some size on as fast as possible with the least amount of effort/time spent. This is the wrong approach to take and it's good advice to train, eat, and research. Fitness is a marathon not a sprint. It's a lifestyle change and the consequences of messing with your hormones can be severe. The sides are not worth being swole for 6 months which is what happens to the vast majority of 18-20 year old anabolics users. It's a phase for most and the uninformed decisions made during that phase can haunt you for years to come. Hormone imbalances can change your life and are nothing to play with. When I think back to all of the guys I knew when I was 20 that jumped on a cycle and got swole right after high school, only one is still in shape. the rest haven't been to a gym in years. 1 of them had a bad recovery that almost cost him his new marriage. I just ran my first cycle at 28 after training for 10 years and researching since my early 20s. I have studied the ins and outs of every aspect because training is a hobby of mine and I have a genuine and scientific interest in it. I can't tell you how glad I am that I waited until I was knowledgeable. Not just about the substances themselves but more importantly training and eating. It's not what anyone wants to hear but all the guys telling you to wait are guys that care and they are looking out for you.
Now, to answer your questions. Your endo system won't be affected more at 20 than it will be at 30. I wouldn't call you a dumba** as you stated. You took your first step in the learning process by asking informed personnel. However, hearing you say I don't know how this Sh*t works, is your answer to why most guys tell users under 21 to wait. Also, AG is exactly right. A 20yo is most likely not very developed and nowhere near genetic potential. As for SARMS. I am personally a fan. Some guys aren't. In my opinion they are NOT safer. They are milder. For example a .22 pistol is not safer than a .45, it is weaker.