I have no doubt that Shlomo HaMelech, King Solomon, wrote שיר השיריםת Shir HaShirim, Song of Songs in his old age.
First of all, in King Solomon's youth, he was involved with many women. He admits it in קהלת Kohelet, Ecclesiastes.
ח כָּנַסְתִּי לִי גַּם-כֶּסֶף וְזָהָב, וּסְגֻלַּת מְלָכִים וְהַמְּדִינוֹת; עָשִׂיתִי לִי שָׁרִים וְשָׁרוֹת, וְתַעֲנֻגוֹת בְּנֵי הָאָדָם--שִׁדָּה וְשִׁדּוֹת.
8 I gathered me also silver and gold, and treasure such as kings and the provinces have as their own; I got me men-singers and women-singers, and the delights of the sons of men, women very many.
ט וְגָדַלְתִּי וְהוֹסַפְתִּי, מִכֹּל שֶׁהָיָה לְפָנַי בִּירוּשָׁלִָם; אַף חָכְמָתִי, עָמְדָה לִּי.
9 So I was great, and increased more than all that were before me in Jerusalem; also my wisdom stood me in stead.
י וְכֹל אֲשֶׁר שָׁאֲלוּ עֵינַי, לֹא אָצַלְתִּי מֵהֶם: לֹא-מָנַעְתִּי אֶת-לִבִּי מִכָּל-שִׂמְחָה, כִּי-לִבִּי שָׂמֵחַ מִכָּל-עֲמָלִי, וְזֶה-הָיָה חֶלְקִי, מִכָּל-עֲמָלִי.
10 And whatsoever mine eyes desired I kept not from them; I withheld not my heart from any joy, for my heart had joy of all my labour; and this was my portion from all my labour.
For King Solomon, writing Kohelet was a crucial stage in his תשובה Tshuva, repentance. Afterwards he wrote Mishle, and only after that was he sufficiently "cleansed" to be able to write something as multi-leveled as Shir HaShirim.
Especially if you're used to Artscroll's English
Song of Songs Chapter 1
א שִׁיר הַשִּׁירִים, אֲשֶׁר לִשְׁלֹמֹה.
1 The song of songs, which is Solomon's.
ב יִשָּׁקֵנִי מִנְּשִׁיקוֹת פִּיהוּ, כִּי-טוֹבִים דֹּדֶיךָ מִיָּיִן.
2 Let him kiss me with the kisses of his mouth--for thy love is better than wine.
ג לְרֵיחַ שְׁמָנֶיךָ טוֹבִים, שֶׁמֶן תּוּרַק שְׁמֶךָ; עַל-כֵּן, עֲלָמוֹת אֲהֵבוּךָ.
3 Thine ointments have a goodly fragrance; thy name is as ointment poured forth; therefore do the maidens love thee.
ד מָשְׁכֵנִי, אַחֲרֶיךָ נָּרוּצָה; הֱבִיאַנִי הַמֶּלֶךְ חֲדָרָיו, נָגִילָה וְנִשְׂמְחָה בָּךְ--נַזְכִּירָה דֹדֶיךָ מִיַּיִן, מֵישָׁרִים אֲהֵבוּךָ. {פ}
4 Draw me, we will run after thee; the king hath brought me into his chambers; we will be glad and rejoice in thee, we will find thy love more fragrant than wine! sincerely do they love thee. {P}
ה שְׁחוֹרָה אֲנִי וְנָאוָה, בְּנוֹת יְרוּשָׁלִָם; כְּאָהֳלֵי קֵדָר, כִּירִיעוֹת שְׁלֹמֹה.
5 'I am black, but comely, O ye daughters of Jerusalem, as the tents of Kedar, as the curtains of Solomon.
ו אַל-תִּרְאוּנִי שֶׁאֲנִי שְׁחַרְחֹרֶת, שֶׁשְּׁזָפַתְנִי הַשָּׁמֶשׁ; בְּנֵי אִמִּי נִחֲרוּ-בִי, שָׂמֻנִי נֹטֵרָה אֶת-הַכְּרָמִים--כַּרְמִי שֶׁלִּי, לֹא נָטָרְתִּי.
6 Look not upon me, that I am swarthy, that the sun hath tanned me; my mother's sons were incensed against me, they made me keeper of the vineyards; but mine own vineyard have I not kept.'
If this really is the spiritual allegory that chazal, our wise men, claim it to be, it must have been written by the post-Kohelet Shlomo HaMelech.
It's possible that King Solomon wrote it as a personal plea to G-d to accept Tshuva, repentance.
These are my thoughts after we read Shir HaShirim this morning during Shabbat prayers.
Chag Kasher v'Sameach
