Hi

You can apply for EWS certificate irrespective of your age dear, so it doesn't matter even if you are more  than 18 years old dear.

You can apply for E W S certificate only if you are eligible for it

For your reference,

EWS eligibility is provided below:-

-------------------) you shouldn't be covered under the scheme of reservation for OBCs ,SCs and STs

-------------------)your gross family annual income from all sources i.e. salary, agriculture, business, profession, etc. must be less than 8 lakh rupees

-------------------) irrespective of the annual family income one won't be eligible for ews reservation if his /her family owns

a) agricultural land of 5 acres and above

b) Residential area of 1000 sq ft. and above

c) Residential plot of 100 sq. yards and above in notified municipalities

d) Residential, plot of 200 sq. yards or  above 200 sq. yards in  the areas which are those other than the notified municipalities.

You can check the same at its official page the link is provided below

https://dopt.gov.in/sites/default/files/ewsf28fT.PDF

Thank you