I have 3 Hibiscus rosa-sinensis that are about 42 years old.

They get full sun on a gentle slope in my backyard. I've pruned them back about a third three times in the 10 years I've lived in the house. I live a block from the ocean and there is no irrigation to their area. Their appearance is underwhelming. Can they be rejuvinated? If they need deep feeding, which product is recommended? Or is it their time?

Thanks for the help.