In SSC tax assistant can get promotion after 8-10 years of service. The first promotion in SSC for tax assistant will take 8-10 yeran and second promotion will get 3-4 years.

Promotions depend on the number of vacancies available in that region and you have to pass the departmental examination meant for.
I: TA (Customs) can apply only for promotion for PO and Examiner.
II: TA (Excise) can apply only for promotion for Excise Inspector.

Candidates first promotion will take atleast 8-10 years, after which, their G.P. will be Rs. 4200 and they will become a Senior Tax Assistant, or commonly referred to as Executive Assistant.

The next promotion can be achieved 3 to 4 years after that, which will be an Excise Inspector, A Group-B Gazetted Post.

Pay Band – Salary of Tax Assistant(TA) in CBEC is in the pay band 5,200-20,200 with grade pay of Rs. 2,400.

Tax Assistant in CBEC can have three types of postings namely
Excise
Service Tax
Customs

Promotions & Department Exams
Senior Tax Assistant
Excise Inspector/PO/Examiner
Superintendent/Appraiser (Group-B gazetted)
Assistant Commissioner
Deputy Commissioner (+4 years)
Joint Commissioner (+4 years)
Additional Commissioner of Income Tax (+5 years)

Tax Assistant is usually promoted to Senior Tax Assistant in 3 years after his joining, which is 4200 GP post, and can become Excise inspector/PO In next 3-4 years, means in total 6-7 years he can reach upto 4600 GP.

Age:
Age Limit for UR is 18-27 year.