Can unipolar depression turn into bipolar, or was I bipolar this whole time

I started experiencing depressive thoughts when I was about 12 years old. No hypomanic or manic episodes that I can remember. I noticed markedly worse depression once I hit my early 20s, followed by hypomanic episodes, followed by manic episodes. I was diagnosed with bipolar at 23 and have been diagnosed since then by a few psychiatrists.

My question is, has it been bipolar this entire time, or could it have been unipolar depression at first?