Well many people drop a year to prepare for jee mains and advanced so there is no harm in taking a year drop. Sometimes, it is difficult to manage to study school level syllabus along with the syllabus of jee, especially jee advanced. Jee mains does not need a year drop if you study in +2 by heart but advanced needs some tome to prepare. 

Talking about upsc ( exam required to become ias), its given age limit is 32 years for general , 35 for obc and 37 for sc/st. Even for general , it is possible to crack upsc within the age limit. If suppose you're 18 years old now, you'll be 23 by the time you complete your degree. Still, you'll have 9 years minimum to prepare for upsc. That's enough time.

Hope this helps!