There always is some confusion in the entry age ( minimum age as of 21st March for that year's enrollment or upper limit). States have different rules. For instance in Karnataka, in most private unaided schools, the entry age for Class-1 is 5 years and 10 months and the entry age for LKG is 3 years 10 months.
There is some controversy in Delhi, the official age is minimum three years as of 31st march 2017 to enter LKG. some schools in Delhi follow the upper age limit for entry to upper age limit for entry into LKG is 4 years, UKG is 5 years and 1 standard is 6 years.
So it's worthwhile to check the website of the education department and the schools you wish to apply, now itself so that you have an idea whether to put the child in a Pre-nursery or not.
Many schools have few years of activity early that 1 standard
Pre-Nursery or Toddler Group - 2 years and above
Nursery or Pre-KG - 3 years and above
KG (LKG,UKG) - 4 years and above
Then Class/Standard-1 - 5 years and above
Pre-school is not mandatory as long as the child is eligible for I standard admission and the entrance tests that many school hold before admission into the respective classes. So my suggestion is, it would be better if the child completes LKG, UKG and then is ready for 1 standard.
based on your information, the child should be ready for enrollment for Pre-Nursery in 2018-19, LKG in 19-20, UKG in 20-21 and 1 standard in 21-22. Usually applications are given during or announced in January for that year.
Given the difficulties people face now, i would suggest that you decide whether you are going ahead for pre-nursery, then shortlist 4-5 good schools in your area and then approach them directly now itself to avoid confusion and to ensure that the entry age is met (a few months of age either side, the child can or cannot start that particular academic session).