With an 18 year old Amazon and understanding their Body Language, there is no reason to wait. As long as you are paying close attention to the Body Language and moving at the rate allowed by the Amazon -- you can work with Step-Up on Day One.
As you have found, it's better to begin without the other animals around.
Any bird needs time to adjust to a new home and to a new person (no matter the age). Individuals still have to establish that trust before trying to handle a parrot without the bird's "person" around to protect the bird (no matter the age of the parrot). It's possible it could work on day 1 if the bird likes someone a lot and feels that connection. I do agree with what Sailboat said about reading body language, but some of that observable body language is produced as a warning/in reaction to stress etc. That means if you are seeing it, you are often causing it and if you are causing it, then you are not necessarily going to be building that association between yourself and low-stress/positive activities. If your bird wants to initiate etc, you can try, but I'm just saying you don't want to keep testing something if you get threatening body language, because that is a sign that you are stressing out the bird. If you do get the bird to step up, you are then dealing with a bird that you do not know (who may try to climb up your arm etc). This is why it's best to get to know the bird a bit.
It's not just about avoiding the bite, it's also about proving yourself to be trustworthy and respecting boundaries, while keeping the bird relaxed/calm when you are around.
When I got mine as an adult, she knew how to step up etc, but would not for a long time.
If you can safely do it, I would try to let your bird explore its cage with the door open (with supervision) so that it doesn't decide that going in means getting locked up. Mind you, chasing the bird to put it back in will not go over well...nor will the bird flying into a window or out a door (so keep all of this in mind). You may have to wait for your bird to go back in, so don't try this unless you have time...