The protagonists of The Swiss Family Robinson are, of course, the members of that family. There is a couple, and they have three sons with them. The sons are Fritz, Ernest, Jack, and Francis.
Fritz is the oldest of the children in the family. We know that he is 14 years old at the time of the shipwreck. The next son is Ernest. He is 12 years old when the book begins. The third son is Jack. He is 10 years old at the start of the book. Finally, there is Franz, the baby of the family. He is 5 years old when the family is shipwrecked on the island.
We can get all of this information from the first chapter of the book. There, the narrator (who is the father) describes the people who are in the improvised boat that will get the family from the wrecked ship to land. He says that, next to his wife,
was Franz, a sweet-tempered, affectionate little boy, nearly six years old. Then came Fritz, a handsome, spirited young fellow of fourteen; the two centre tubs contained the valuable cargo; then came our bold, thoughtless Jack, ten years old; next him twelve-year-old Ernest, my second son…
This gives you the ages of all of the children of the family. The parents’ ages are not given in the book, though we would expect that Mrs. Robinson would be in her late 30s since her oldest son is 14. It is likely that the father is somewhat older, as men in those days often married younger women. However, that is just speculation. We only actually know the ages of the children.