Ideas for testing reaction times
You need to make sure that the method you decide to use is suitable for all ages. For instance, it is not a good idea to ask elderly people to stand suddenly, or to catch a ball - as this may not be easy for people with arthritis or whose movement is restricted. You should also try to ensure that the method is suitable for people with failing vision or whose hearing is impaired. This is not an easy task.
This method does not actually give you a time - but it is easy to set up and it does give you something measurable. The end of the tape is held against the START LINE by the tester. As the tester releases the tape (and says "NOW!"), the person being tested stops the tape with their finger. The person who lets the weight fall the furthest is obviously the slowest to react.
You could use a computer-based testing method - such as this one.
This method is even easier to set up. The tester holds the ruler so that the bottom edge is level with the fingers of the person being tested. The tester releases the ruler (and says "NOW!"). The person being tested closes their fingers to catch the ruler. It is wise to use a metre rule.