The class most people this age quietly avoid is not too hard for them. It is aimed at something they no longer need, and I would rather point you at the one that is.
What actually matters now
Three things, in order.
Balance, because falls are the event that changes everything, and balance is extremely trainable at any age and declines fast without practice.
Strength, particularly legs and grip, both of which predict independence better than almost any other measure.
Bone loading, which requires being upright and bearing weight rather than lying down.
Flexibility is on the list, well below those three, and it is the one most people over sixty think they are supposed to be working on.
Which points to standing work
Standing positions, held. They train all three at once, and they do it with no impact and no requirement to get up and down off the floor repeatedly.
That is why alignment-focused classes suit this age so well. It is not because they are slow. It is because they spend most of their time doing precisely the thing that matters and holding it long enough to count.
What to be more careful with now
Deep flexion of the spine under load, particularly if bone density is a question you have had answered unfavourably. Tell me before the class, not during it.
Inversions, if blood pressure or eyes are a factor.
Anything that requires you to get from standing to the floor and back quickly and repeatedly, at least at first. That is trainable, and it is a goal rather than an entry requirement.
The thing nobody says
You will improve. Not maintain, improve.
Untrained people at sixty and seventy make faster relative gains than twenty-five-year-olds who already train, because the starting point is lower and the body has not stopped responding. This is very well established and almost nobody believes it about themselves.
Starting
Tell me your age and your history at the door and ask what the class involves. Then take something slow and technical rather than something billed as gentle. They are not the same, and the technical one is usually your better hour.