Here is a link to an article dealing with this question: click here. The post doesn’t offer a specific age, but presents some important things to consider.
Personally, I lean towards encouraging those younger than 12 to wait. I don’t have a direct scripture for this, but like the post above, think too many kids below 12 have not fully formed their own convictions. They typically don’t understand the meaning and symbolism of baptism and membership in the church.
Pastorally, I will baptize those younger than 12 if they ask themselves, their parents are supportive and think they are giving good evidence of belief, and they can articulate their faith and the Biblical meaning of baptism. I recognize this is subjective, but my goal is not to withhold baptism to a genuine believer or to administer baptism to an unbeliever.