I think 7000-year-old Galadriel and 6500-year-old Elrond at the end of the Third Age just haven't gotten to their "third cycle" yet. Having your children grow up, or becoming a grandmother, evidently isn't the end of the "second cycle". Even with Gwindor, his face probably looked old but I don't imagine him sprouting a beard. At the end of the Third Age, Cirdan is at least 10,655 years old! If he lived at Kuivienen before the Great March, he was more like 10,900.
The half-elves were a special case. Elrond and Elros and all their half-elven ancestors were born before Earendil and Elwing came to Valinor. At that time Manwe says what I interpret to mean "All half-elves have been mortal until today, but from now on I will grant them a Choice." Their sons were 10 by then, and probably kept growing at the mortal rate, but after they were grown I think the Valar put them into a sort of non-aging limbo so they wouldn't get old before they had a chance to make their Choices.
Afterwards, my understanding was that Elrond's children grew and remained young exactly like Elves do, and would remain that way until he sailed West. But I expect that Elrond will have beard by the time he's 11,000 years old or so.
On the other hand, I think I saw in another thread that you made Celeborn one of the Elves of Kuivienen, instead of the grandson of Elmo. So then you need to explain why he doesn't have a big beard in the LotR, despite being the same age as Cirdan or even older.