You shall observe this rite as a statute for you and for your sons forever.
The Passover was given in covenant succession, and was intended as a means of teaching the children what God had done for his covenant people (Ex 12.26)
"The whole congregation of Israel must celebrate the passover."
The Institution of Passover did have an exception that was clearly stated: non-circumcised families of foreigners could not participate. If they converted, they could join the meal. But age is never a reason for exclusion.
1 Corinthians 10.1-6
For I want you to know, brothers, that our fathers were all under the cloud, and all passed through the sea, 2 and all were baptized into Moses in the cloud and in the sea, 3 and all ate the same spiritual food, 4 and all drank the same spiritual drink. For they drank from the spiritual Rock that followed them, and the Rock was Christ. 5 Nevertheless, with most of them God was not pleased, for they were overthrown in the wilderness. 6 Now these things took place as examples for us, that we might not desire evil as they did.
POINT #1 - Notice here that Paul wants us to see NT theology in the Exodus - that all covenant members were Baptized, and in correspondence, all covenant members were communing with Christ himself in the Manna and Water from the Rock.
POINT #2 - Examination for sin was necessary in the OT times when children were clearly included. Paul is (v.6) actually using this to teach examination to the Corinthian church. So examination is not new, and cannot newly exclude children (1 Cor 11.27-32).