NATA

Please refer to the link for complete details: http://www.nata.in/.

NATA 2018 results have yet not been declared. You can await notifications when application begin in February next year.
https://www.shiksha.com/architecture-planning/exams/nata/dates

There is no lower age limit to take NATA 2018. Without any limitation, you can give NATA exam.

Eligibility for B.Arch:
Candidates should have passed 12th standard or equivalent examination with a minimum aggregate of 50% marks with Mathematics and English as subjects of study. 10+3 Diploma (any stream) recognized by Central/ State Governments with 50% aggregate marks.

Some of the popular books for preparation for NATA entrance exam are as follows:
A complete Self Study Guide for B.Arch. Entrance Examination 2018 By P K Mishra
NATA Guide to B. ARCH Entrance Examination with solved papers 2013 -16 By GKP
Steps to Architecture NATA and JEE Mains Entrance Exam Guide By R Gupta
NATA & B.Arch Question Bank By Ar. Shadan Usmani

Please use the link provided below for details regarding admissions in Manipal University, Faculty of Architecture for B.Arch. You should be able to download their brochure which contains details regarding NATA eligibility and admission process to secure a seat in Manipal for B.Arch.
https://www.shiksha.com/architecture-planning/course/b-arch-manipal-university-faculty-of-architecture-192900

Please refer to the link provided below for details regarding admission procedures to gain entry into a course in architecture in CCA through NATA score. You should be able to download their brochure which contains important details like last application dates, admission procedure, the application process to be followed and fee structures.
https://www.shiksha.com/college/chandigarh-college-of-architecture-3568

Eligibility Of the Bachelor of Architecture [B.Arch.]
The duration of the course is 5 years and to get admission into B.Arch. the student must be fulfilled the following conditions:
10+2 or equivalent with minimum 50% marks along with compulsory subject of mathematics.
10th + 3 years diploma in any stream from a recognized university/ board/ institution.
Cleared NATA (National Aptitude Test in Architecture) with the 80% marks conducted by Council of Architecture.

No, if you want to pursue B.Arch from SPA-Delhi then you have to appear in JEE Mains paper 2 which is especially meant for architecture and planning.