The past tense of dwindle is dwindled.

You can use "dwindle" in a sentence when describing something gradually decreasing in size, amount, or intensity. For example, "The water supply continued to dwindle during the drought" or "Her savings began to dwindle after she lost her job."

How about "The vocabulary of America's Youth is starting to dwindle."? ;)

"Dwindle" means for something to gradually grow less or smaller

Dwindle means to slowly get smaller. The amount of water had started to dwindle because of the extremely hot summer's day.