First, your not getting as hard as when you were a teenager could be due to lower than normal testosterone levels, some medical condition and/or medications you are taking for it. Your frequency of masturbation might be so often, that your penis "objects" and won't get as hard, or that as a teenager, your penis got really hard and in your twenties, doesn't get quite as hard, although usually it would.
Daily masturbation would normally have no effect on the erections of a teenager or twenty-something year old. During that time of your life, you guys normally come quicker, but can do it over and over.
The reason the head/glans does not get as hard as the rest of your penis when you have an erection, is quite simple and based on anatomy. The penis is composed of three erectile bodies. One on each side, and one in the middle on the bottom.
The greatest mass of the penis is formed by the two corpus cavernosa bodies which form the sides and top, and the while the urethral portion (on the underside) and the head are formed by the corpus spongiosa.
Now that you know this, it is also easy to understand why the penis is more sensitive on the glans and underside and why rubbing the urethral areas of the underside of the penis feel so good to the man.
The glans and urethral containing areas of the penis generally do not get as hard as the rest of the penis.
Have fun with your cocks, guys. I know I will!