I apologize for this delayed reply to your gardening question and hope it is still helpful to you.
I would not think that age of the plants would matter. Failure to set fruit is usually due to a problem with the flower getting pollinated or with insects inside the flower damaging it and causing it to abort. I would either give them more time or try planting some new eggplant plants to see if these new plants set fruit. You might also consider calling your County Extension Office to see if they have any additional thoughts on this since they are more familiar with your local conditions.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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