Being baptized is a public expression of faith. Although many denominations practice infant baptisms, my belief is that these are more in line with being a Baby Dedication, because all examples of baptisms in the Bible [that I'm aware of] include someone coming by choice, following the instructions to repent, and be baptized.
If you are old enough to understand salvation and the consequences of sin, you are old enough to receive the gift of salvation and give testimony to that fact by baptism.
Anyone that is a baby or toddler can be dedicated by the church to the Lord. But my own children 5 and 6 asked me to get them baptized. They knew who God was and who Jesus was and we would read the bible so that they had an idea. They also took a small class on what does baptism mean. Baptism is important and should be done. If you feel you know who the Lord is, that he's important to you and you do some studying on baptism, you should be able at any age after 5 to be baptized.
This is dependent on your church's and denomination's particular beliefs. Some faiths, like Catholics, baptize (sprinkle) as infants. Some faiths, such as Baptists, do full immersion baptism and believe a child needs to reach an age of acceptance and understanding for right and wrong.
Well its any age really you don't have to be 3 or some thing to be baptised you can be 81 for any one cares!