18 Mbps is certainly not enough today to play most of the modern online games. Today to play famous online titles you will need at least 30 Mbps of speed for a smoother gaming experience. For most MOBA and other online games, anything above 30 Mbps is considered good. If you also stream along with gaming then you will require a bandwidth of about 100 Mbps.
If you are not a hardcore FPS or MOBA player and only want to do casual gaming then 18 Mbps will work just fine for you but do not expect a very smoother gameplay
When it comes to online gaming it is not only the bandwidth that matters there are other aspects that you need to see for a lag-free online gaming experience. It depends on the type of your connection, latency, ping rate, and server.
You can know more about Mbps for gaming here and see how you can have a lag-free experience