Concept:
Universal Adult Suffrage is guaranteed under Article 326 of the Constitution of India. To alter the eligibility criteria for participating in general democratic elections, a constitutional amendment under Article 368 must be enacted by the Parliament of India.
Step 1: Examining Article 326 and its historical amendment.
Originally, the text of Article 326 stated that every citizen of India who was not less than 21 years of age was eligible to be registered as a voter. In the late 1980s, the political leadership decided to lower this age boundary to encourage the participation of youth in democratic processes.
This structural change was introduced via the 61st Constitutional Amendment Bill, which was introduced in 1988 and came into force in 1989 after receiving ratifications from state legislatures. This amendment officially substituted the words "twenty-one years" with "eighteen years" in Article 326.
Step 2: Checking alternative options to ensure accuracy.
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45th Amendment (1980): Extended the reservation of seats for SC/ST categories in Parliament and state assemblies.
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56th Amendment (1987): Formulated special provisions for the statehood of Goa and fixed the composition of its assembly.
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69th Amendment (1991): Awarded a special constitutional status to the Union Territory of Delhi, designating it as the National Capital Territory (NCT) of Delhi.
Thus, the 61st Constitutional Amendment is the correct constitutional instrument that modified the voting criteria.