There is no age restriction. All are welcomed.
The Catholic church never restrict salvation and baptism to adults alone. The Catholic church believes in Infant Baptism, this site explains it. A passage from the site -
More detail is given in Luke’s account of this event, which reads:
"Now they were bringing even infants to him that he might touch them;
and when the disciples saw it, they rebuked them. But Jesus called
them to him, saying, ‘Let the children come to me, and do not hinder
them; for to such belongs the kingdom of God’" (Luke 18:15–16).
Altar Call is practiced by many churches, especially in Evangelical churches. Christianity in general welcomes every one to Christ, young or old, rich or poor, black or white. During the Altar Call, the preacher would invite the audience to repent and give their life to Jesus and proclaim publicly by coming forward. Many times, there are situations where children would come forward with sincere heart and with tears. The people usually responded to this by giving thanks to God.
As an example, I have seen children below 10 years who received Jesus Christ during the Altar Call in Assemblies of God Church.