I couldn't find a direct textual support for either of these hypotheses, but I think they are at least textually consistent:
1 Jennifer and Anissa could have developed their powers at the same age, but Anissa didn't notice until later.
The default state of Jennifer's powers is very obvious. Without even trying she can glow and when she totally loses control she can become incorporeal. By contrast, Anissa needs to breathe in a certain way and hit something or be shot. If she's not actively pushing on something, there's no obvious sign of her powers. Perhaps she had simply never stumbled on the correct breathing before.
2. Age may not be enough. They may need to encounter some sort of trama after reaching a certain age to activate their powers.
This hypothesis is supported in that many of the people in the show who develop powers seem to encounter some trama shortly before finding out about them. The girls are kidnapped, Jefferson saw his father murdered, and the NSA inflicts painful experiments on people to test their powers.
Of the two, I think the first hypothesis is more likely. In episode 1 we hear that Anissa was involved in a protests against the 100 that turned violent and led to her arrest. If trama were all that was needed to unlock her powers, a violent protest seems like it would have been enough to activate them.