LOL, good on Phyllis.
Linda, there is no particular age as each plant can grow at a different rate based on its health and the environmental conditions.
The larger the leaf the better. The plant grows from the inside out which means that the older leaves are on the outside and are lower than the ones on the inside. The longer you wait, the more you will get from your plant, but a burned finger will needs it even if the leave is rather small.
You might want to have at least two plants to use so that they can continue to grow as they are used.