In Jainism, a child is generally not expected to perform formal puja immediately after birth.

Simple rule:

• A newborn does not do puja.
• As the child grows, learns shuddhi, vinay, bhakti, and proper vidhi, they may start participating in puja.
• After becoming able to understand and follow discipline respectfully, a child can begin simple puja with guidance from parents, gurus, or elders.

Practical understanding

• Infants and very young children: only brought near the temple respectfully; they do not perform puja on their own.
• When the child can sit properly, remain clean, and follow instructions: they may begin with very simple devotion, like joining in namaskar, folding hands, and observing.
• Formal dravya puja should be done with proper purity, conduct, and understanding of Jain vidhi.

Important Jain principle

• bhavna,
• shuddhi,
• ahimsa,
• and proper conduct.

So the child should begin puja when old enough to do it respectfully and correctly, not by a fixed age.

If you want, I can also explain this separately for Digambar and Shwetambar traditions.