Sounds like you have filled in the application wrongly but you have a correct valid original birth certificate with you. There should not be a major issue as it is not lying about your age but an error in writing/ form filing. But you need to update the authorities. As far as RRB scale-1 eligibility goes, for the general quota, the age limit is above 18 years and below 30 years as of September 2017/year of the announcement.
You would have applied this online? Then you cannot correct Date of birth once you submit the form along paid your fees.(FAQ 10:http://ibps.sifyitest.com/rrbgas1jul17/uploads/loadpdf.php?file=k7m5p+fQ15e6zM3awtyW1N7Sm5x9sc/KYaao1bWrpok=&t=yqrCZuPFyg==#toolbar=0&navpanes=0). You can also highlight your mistake at the time of the Common interview, after the two qualifying exams.
Please contact the Candidate queries are entertained on 1800222366,
Then, it's better you have to write to them along with the attested copies of your date of birth certificate and a printout of your primary application form and explain your mistake.
IBPS House, 90 feet, D.P Road
Near Thakur Polytechnic
Off Western Express Highway
Khandivali (E). Mumbai 400101.
you can also contact IBPS blog sites who can help you also (https://ibps.blog/about/ mail:email@example.com)