Final answer:
The comparison in the sentence highlights the unfairness of not allowing younger teens to vote based on assumptions about their maturity and responsibility.
Step-by-step explanation:
The effect of the comparison made in the underlined sentence is to highlight the irrationality of not allowing younger teens to vote based on the assumption that they will not vote responsibly.
By comparing it to not allowing a child to play the piano for fear of them making a mistake, the sentence suggests that such a restriction is unfair and hinders the development of maturity and responsibility in young teens.