Since becoming an adult, I feel my child's speech has gradually decreased.

The ability to talk is maintained and grows through the accumulated experience of conversing with people. When someone's community becomes limited as an adult, the very opportunities for conversation decrease, and speech tends to decline too. Here are four suggestions. 1) Join a new community connected to something they like or a hobby. 2) Have them take on a "role" inside or outside the home (a need to talk naturally arises). 3) Converse while writing. 4) Plan activities among the adult members of a parents' group where the parents don't intervene. And above all, what matters most is conversation time with "a listener who speaks clearly, understands accurately, and waits." The experience of listening and imitating grows speech at any age.