I rehydrated and pitched yeast Sunday at noonish. Here it is Monday at 4:00 pm and realized I forgot to aerate. Fermentation temp is about 58 degrees. I pitched at 68 degrees. There is no activity in air lock. Is it too late to aerate?
I wouldn't aerate. It could cause oxidation. When you poured the wort into the fermentation bucket did you let it splash a bit?
yes, it's too late. Aerating is important but not entirely necessary. If anything, your temp is too low, try and bring it up to 64 or 65.
I don't know for sure (I use liquid yeast), but I thought I saw that aeration was not necessary with dry yeast. They have everything need to get started and reproduce.
You might want to try searching the past threads.