The Laws of Prayer (89)

1. One who responds Amen should not raise his voice louder than the one who made the blessing, as it says in the verse “Make Hashem great with me and we will exalt His Name together!”, and it would appear that the same is true regarding responding to Barechu and to Zimun (the blessing recited when three or more people have eaten together). If he raises his voice in order to encourage other people to respond Amen, it would appear to be permitted. (Shulchan Aruch and Mishne Berurah 124)

2. One should respond Amen after a juvenile makes a blessing. However, one should not respond Amen when a witless person makes a blessing. (ibid)