Mine are about a year old, I guess. I have one in a 10g and one in a 15g. Before I got them, I had a little GSA on the 10g tank walls. I had to scrape it off every other or third water change. It really wasn't bad.
I put one in the 10g while it had a little GSA on the walls. The rest of the week, I could see where it had been eating. That next Sunday was water change day and I scrapped the rest of the GBA off. When I refilled the tank; that little fellow went round and round the tanks walls with it's mouth stuck on the tank was. It was in a verticle position when it was going horizontally (or sideways) round and round the tank. It was really funny watching it do this.
The next couple of water changes, I scraped the little bit of GSA off that it hadn't eaten. Everytime after I did this; the little fellow went round and round the tank.
Now, there's not a spot of GSA. The little fellow is doing it's job well. It's not showing any aggression like you mentioned with yours that's 5 years old.