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It should be flowering at this age, so there is something keeping it from blooming. There are several things that can cause this. This article will help you with determining what it is:
https://www.gardeningknowhow.com/plant-problems/environmental/plant-not-blooming.htm

There may be a pest problem. Check the plants closely as some pests can be small. Treating them wtih neem oil often helps. It's safe for eliminating most insect pests and is a great fungicide as well.

This article may also help: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/answers-to-what-causes-hibiscus-leaves-turning-yellow.htm

It is poisonous to some animals, but is not lethal. It normally will make animals that are affected by it sick to their stomachs.

It sounds like a fungus. They can treat the plant with a fungicide and that will get rid of it. Improving airflow around the plant will stop it from coming back.

Try neem oil. It's good for many types of insect pests. Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

They are either the pests themselves, like scale, or damage from the pests. Many pests that can affect hibiscus do not move much once attached to the plant. Before bring it indoors for its dormant period, spray it down with jets of water to knock off as many as the pests as you can and then treat the plant with a pesticide. An organic or chemical pesticide will work, which you use is up to you. This will help keep the pests from spreading to your other plants indoors.