Tell her about all the great things about younger men. For example that they have less baggage than older men, they are more energetic and have more life in them, they know not to take everything too serious, they are more eager than men her age etc. If you are really risky tell her that they have bigger cocks lol.
Basically let her know that she should atleast try one younger man while she is single as you've heard from women online in her position of how great they can be. If you keep pushing this idea to her then you will see that eventually she might take that offer seriously.