( d.) Any man or woman of any country, religion or caste who embraces Sikhism and solemnly undertakes to abide by its principles is entitled to ambrosial baptism.The person to be baptised should not be of very young age; he or she should have attained a plausible degree of discretion.
The above indicates that there is no formal age for becoming a baptized Sikh. It can be carried on any time when a Sikh thinks he or she is ready to fulfill the duties & responsibilities of being an Amritdhari Sikh / Khalsa.
Like Soul Jyot said there is no age requirement but when you feel that you are ready. This also leads me to ask a question is is right to give Amrit to a newborn child like if both parents are Amritdhari, or should they wait till the child can make his or her own decision?