Alright, so I have two artists that I'd like to compare (and hope that people here know who they are)

1.) Cat Stevens
2.) Van morrison

for those of you who do know them, I've noticed that when Cat started singing in the 60's and then again in the 70's after he got out from the hospital, his voice didn't really change much... In fact, it's almost indistinguishable to find a difference in his voice from his 1967 album compared to his 1970 album. That's 3 years!

As for Van Morrison, you can definitely tell there is a difference from when he recorded one album in 1968 on to his next one which was released in 1970

You can hear how his voice has gotten 'deeper' in a sense, or in this case just more mature sounding...

So my question is, how would I go on about doing that? I've heard that drinking alcohol does get your voice to go deeper (correct me if I am wrong, please) and I also think that it can damage it as well (again, correct me if I'm wrong)

I'm training my voice right now, so when I'm 25, I'll have at least a decent voice but I don't want to sound the same always!

So how would I go on about if I want my voice to change into a more mature sound? I know it takes time but is there anything else?
three years isnt that long
But boys will be boys and girls have those eyes
that'll cut you to ribbons, sometimes
and all you can do is just wait by the moon
and bleed if it's what she says you ought to do
Alcohol can definitely affect your voice. Whether or not you consider that a negative depends on the implications and the kind of person you are. Chris Cornell is an example of someone who watered down their voice and range due to alcohol. If you compare his singing on the first Audioslave album to how he sounds now back with Soundgarden, he's hitting high notes with ease again. He actually sounds better live than towards the end of Soundgarden's tenure, which goes to show how dramatically alcohol can alter your tone.