When a Japanese-language sentence is written out in all katakana, it's supposed to be either stilted, robot-like speech or something to simulate ALL CAPS.
How is that with hiragana? Does it make te text look like it was written by a nine-year old when all kanji are written phonetically?
Are there nuances in meaning when the same word is written in kanji or in hiragana?
Related, but not containing the answer I'm looking for: What does it mean if a sentence is in all-kana?
Extensive use of hiragana by intent will make yourself look immature, childish, unserious, drowsy, cute, innocent, or sometimes less intelligent, depending on the context.
A good but exaggerated example is found here. A very childish character in a game, who is always talking in hiragana.
…ふわぁ…あなただぁれぇー？…ぷろでゅーさー？…えぇー…かわいいー？…あいどる？うんー…いいよー…やるぅー…あいどるやるぅー。…で、なにするのー？…わらうのー？…ふわぁ
Huuuoaaa... who are you? a produucer? eeeh, ... cuuute? an idool? yup... ookay... i do... i do, idol ... so what do i do...? to smile...? Huuoaa....
Combination of 長音符(ー) and hiragana, which is generally prohibited, is also a good way to add extra childishness, as in "とーきょー" instead of "とうきょう" (東京 or Tokyo).
In addition, recently, extensive use of smaller hiragana, when possible, has a similar effect. It has become a popular way of chatting via their smartphones among young schoolgirls (google 若者言葉 or ギャル語 for details and more extreme examples).
See the following example (the last one may be rather extreme):
今日は雨が降っているよ。 (It's raining today.)
=> きょうは、あめがふっているよ。
=> きょーは、あめがふっているよ。
=> きょーゎ、ぁめがふってぃるょ。
It does indicate a more childish audience—or at least that you don't think they can read kanji. It also, interestingly, makes things much harder to read for those who know kanji—doubly so if you don't put spaces in between words or after particles.
Hiragana can also be used in place of kanji at times to allow for the execution of Japanese puns in text as well, since all-kana text requires more context to be clearly understood. Best example of this I can recall is a chapter title from Ranma 1/2 called "ちちと母" where the expected 父 was left in hiragana to provide an allusion to 乳 and the mother's anticipated disapproval of her son's transforming condition.
