We baptise those who have become Christians by choosing Jesus as their Lord and Saviour and want to make a public commitment to that effect.
Go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit [Matt 28:19 NIV]
Repent and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins. And you will receive the gift of the Holy Spirit. [Acts 2:38 NIV]
1 What shall we say, then? Shall we go on sinning so that grace may increase? 2 By no means! We died to sin; how can we live in it any longer? 3 Or don't you know that all of us who were baptized into Christ Jesus were baptized into his death? 4 We were therefore buried with him through baptism into death in order that, just as Christ was raised from the dead through the glory of the Father, we too may live a new life. 5 If we have been united with him like this in his death, we will certainly also be united with him in his resurrection. 6 For we know that our old self was crucified with him so that the body of sin might be done away with, that we should no longer be slaves to sin— 7 because anyone who has died has been freed from sin. 8 Now if we died with Christ, we believe that we will also live with him. 9 For we know that since Christ was raised from the dead, he cannot die again; death no longer has mastery over him. 10 The death he died, he died to sin once for all; but the life he lives, he lives to God. 11 In the same way, count yourselves dead to sin but alive to God in Christ Jesus. 12 Therefore do not let sin reign in your mortal body so that you obey its evil desires. 13 Do not offer the parts of your body to sin, as instruments of wickedness, but rather offer yourselves to God, as those who have been brought from death to life; and offer the parts of your body to him as instruments of righteousness. 14 For sin shall not be your master, because you are not under law, but under grace. [Rom 6:1-14 NIV]
As baptism is a matter of choice and represents a public confession that you are following Jesus we cannot baptise anyone below an age at which they can make that choice and commitment.