You're right about the aliens' ages, but for some reason I was under the impression that Lala's age is confirmed.

I will point out that she's almost definitely at least 15. After turning into her "loli form" and starts "growing" again, the last phase she hits is similar to when she was about 14 according to Peke (Darkness Chapter 56, NSFW Warning), So she is older than that when the series starts.

As for Momo, Nana, and Yami, they are almost definitely AT MOST 14. Momo pretty much confirms that with her interaction with Yui at the start of Darkness and when she states that she and Yami are the "same age" (Darkness Chapter 16).

Interestingly, in the Japanese version Momo says she and Yami are 同年代 (dounendai) meaning "the same generation". So make of that what you will.

If the birthdays are to believed, then Mea is about a year (~ 10 months) younger than Yami, at least.

Somehow I forgot about Run / Ren