I truly believe that children learn from before they are even born. When I am pregnant, I talk to my kids, read to my kids (especially the Bible), and most importantly pray for my kids.
When they are born and are babies I keep it up. I strongly believe that they are taking things in all the time. They are little sponges.
Around 1-2 I really start pointing things out. When we go to the grocery store, I say, "Let's get some yellow bananas. Look at those pretty red apples." I also talk about shapes, colors, objects, and animals. They make tons of cool toys like Baby Einstein blocks that have colors, shapes, and textures that are great.
When they hit around 3, I think it is great to start talking to them about letters (recognition and sounds) and numbers (recognition) as well as Bible characters more.
A lot of the learning comes from reading, talking, and just being in life.
I hope that helps!