Almost certainly you are not too old. We have trained people into their 90s and often in their 70s and 80s. The simple thing to do is for you to come along and give a session a go and see if you enjoy it and find it worthwhile.
Tai Chi moves are all natural but many of them require you to move your body in ways that are unusual to a beginning student (like stepping sideways). Like any new thing, some students take to it quickly and some take longer. The really great thing about learning Tai Chi is that although you generally do it in a group, it is an individual pursuit. So you will take as long to learn it as you need to and that is just fine.
A session lasts an hour and you will be on your feet for that length of time (as a beginner if you needed to take a short break, that would, of course be fine). Other than that there is no fitness requirement for commencing Tai Chi.
Not for us :) Obviously most people progress faster the more often they train but Tai Chi is not a race and as I often say, 'It's easy for life to get in the way' - so if you miss a week or if you miss a year, you will always be welcome to return and carry on.
The rest of your life and then a long time more :)
Seriously, Tai Chi is an enormous field of study. One that you could spend all day every day studying and still only know a relatively little about it. There are different styles and different sub-styles and much much more to learn if you wanted to.
So perhaps a better question is 'How long will it take me to learn enough Tai Chi so that I start to get some benefit from it?' The answer to that is much kinder. You should start to derive some benefit from your first session onwards, and within a few months be able to see that there are changes in the way you utilise your body. You should find that normal movement should start to feel more relaxed and 'smoother'. If you are subject to stress then once you have learned even the first beginners form (Eight Move One Step Form) then you should be able to practice and perform that and find that it helps reduce stress. There is much more, but a chat with Robert Agar-Hutton is probably the most efficient way to answer additional questions.
We will be happy to have you train with us. You will have to learn new things but hopefully some of the things that you learned in your previous style will still be useful. Come along and give it a go and then you will be able to see if our way of doing things meets your needs.