pqr2012;654633..
The problem is renouncing at 18 is possible the only opportunity to renounce USC since India doesn't allow dual citizenship... Which means if you decide to renounce USC (at any other time) you wont have citizenship of any country for certain time frame till you are again eligible to apply for Indian citizenship.
The age of 18 is the EARLIEST age to renounce USC and become IC. It does not mean that the window closes dramatically after the age of 18. Normally, people don't renounce USC first and then have to wait for eligibility to become IC before moving to India. The way it normally works is as follows: Say, a USC/OCI does not want to make a decision or leave US at 18 but at 25. They can continue to live in the US as USC/OCI until they are 25. They can then move to India and go-to-college/job etc (while on OCI). After 1 year, if they decide they want to move to India permanently or convert to IC, they can get IC. Once they get IC, they can renounce USC.
And on taxation, it is not "exceedingly complex" for EVERYONE. it is complex only if you are quite wealthy and derive income primarily from business investments, banking, property etc. For a normal person on a salary, it is just another regular 1040 tax-filing that needs to be done in April. Normally, salaried people pay zero taxes to the US because of double-taxation laws. Whether the benefits of a USC are worth a little paperwork every April should be evaluated by each person based on their situation instead of making a blanket assumption that it will be complex.