Hello Aspirant,

In order to appear in UPSC examinations  candidate must be a minimum of 21 years of age and must not be more than 32 years of age. General Category & EWS: 32 years; 6 attempts. OBC (with certificate): 32 years + 3 years; 9 attempts. SC/ST: 32 years + 5 years; unlimited attempts.

As you're merely 18 now you'll have wait for three more years to appear in the examination. Till then you can continue your preparation.

Hope this helps!!