Spice Jet Airlines — unreasonable ticket charge imposed at a small error

We have booked an air ticket for mum-patna route on[protected] thru online (2 adults, 1 infant) for rs.13320/-. On travel day (03.03.2018) while we reached airport counter they informed that infant had passed 2 years and 3 months, so he need a full ticket. For that ticket they charged rs.12500/-. So this is very unfair by these flight companies acting towards customers. They charged this much huge amount for a 2 years and 3 months old child very cruelly.
I believe our government should look on such matters by these air line companies doing. These are very unfair.