I recommend getting a clicker similar to this one :
https://www.amazon.com/gp/aw/d/B003B...C75M3905D9HR1C
I believe you can definitely train an older dog. To them, it's quality one on one time with you! Make it fun and positive. And don't get frustrated. <--this is key. The second you find yourself frustrated, stop and take a break. The clicker is great. Once you learn to use it correctly, you can train them to do almost anything!! Even now, my oldest is 4 years old and whenever I take out the clicker he gets super excited - like it's playtime with mommy!!
I watched a lot of videos, and like this channel "kikopup". Here's one she does for training sit stay with clicker training.
https://youtu.be/ksBLKi6lj1s