MacBook Air battery drain with High Sierra 10.13.4

I have a 13" 128 GB MacBook Air from 2013, with a 1.3 GHz Intel i5 processor and 4 GB of RAM. My computer's battery life has been excellent, considering that I bought it almost 5 years ago, and even when I first upgraded from OS X El Capitan to macOS High Sierra, I still had decent battery life (about 7-8 hours on average). Anyway, ever since I updated to High Sierra 10.3.4, my battery life has been terrible. In the time it took to type this paragraph up to this point, my battery went down from 92% to 89%. I tried resetting the SMC multiple times, and that didn't work. My display is nowhere near full brightness (and when it is, I have it plugged in). The worst part is when it's in sleep mode. I used to be able to leave my computer in sleep mode for several days, and the battery would go down 3%, if that. Now, it's much worse. I fully charged it last night, and just eight hours later, the battery went down from 100% to 92%, which is when I started typing this. Now, the battery is at 86%. Is it a software issue, or is it just because of the age of my computer?