Is it safe to bring a 5 month old baby to cancun, mexico in december?

First of all, The US government has put a warning out for travel to Mexico, due to collateral injuries and kidnappings based on their ongoing Drug cartel wars. Beyond that, travel with an infant outside of the country requires vigilant precautions. The baby cannot be exposed to any natural sources of water (tap water, vegetables, pools, or natural lakes etc). Minimize sun exposure, and ill persons

I agree, discuss waiting until after the six month visit, with your Pediatrician. Clean bottled water, sun safety and taking U.S. products are advised.