Playing it safe
You may think that when a cat turns 8, she'll no longer be interested in playtime. You'd be wrong. Some cats can go on starting games of hide and seek or chasing toys around until their legs just won't let them anymore, whilst others tend to wind down earlier. You'll have to make a decision, based on what you already know about your cat, as to whether she'll enjoy playing a game with you, humour you, or downright ignore you.
When is enough, enough?
There aren't many visible signs of ageing in cats, but one which you may notice is reduced activity. Some senior cats may begin to prefer the comfort and warmth of that beanbag in the corner to adventures outdoors. The games you invented together, which once she couldn’t get enough of, may start to be just a little too much. However, these changes may not happen until your cat's very old indeed, and some will keep playing well beyond the age of 8.
Keeping her interested
Cats aren't always the easiest animals to keep amused, and this will become even more relevant as age starts to make exercise more difficult. Try to find her a toy she enjoys, but if she's just not interested then respect her decision and leave her be - enforced play isn't fun for either of you!