When did LEGO have a groove in the center tube underneath? I have a bunch of bricks from my childhood, some have grooves in the tubes underneath, some have the old style bubble logo, some have square holes under the plates. I am interested in general ages of each.
I can give you some help, especially if you are talking about 2x4 bricks.  Last year, I embarked on a bit of a history project to trace the design changes in Lego, by looking at the 2x4 brick, from 1949 to today. (Such as changes in the tubes, like the slots you mention!) I talked to several very helpful collectors (some here on Bricks beta!), who gave me lots of helpful information.
I distilled it all down into this article:

http://snakeandboris.blogspot.com/2016/06/a-short-history-of-lego-brick-1949_6.html

(This link goes to Part 2 of the article, because it contains a link for Part 1.)
Keep in mind, though, that not all dates with Lego changes are hard-and-fast.  You will see a few "c." meaning it was AT or AROUND that time--because as Lego changed molds, they didn't do it all at once, across the whole world. They used the old molds for their normal lifespans.
There are even MORE sub-variations than what I have found, and there are links at the end of my post to some other good sources of information.
Good luck! One thing about this hobby I can tell you, there are lots of knowledgeable and kind folks to help you!