There are very few reliable rules regarding hyphenation in English. Very often a hyphen is optional, and the writer has to make a selection. For example, ‘lifestyle’, ‘life style’, and ‘life-style’, are all correct. I would personally write ‘fully developed’ without a hyphen, and it does not change the meaning of the sentence in any way.
The use of hyphens is becoming less common in English because of its complications. Many compound nouns which were once considered incorrect in English without a hyphen, are now considered correct.If you are not sure whether to put a hyphen between words, you should look in a dictionary, or write them without a hyphen.