Of course they know their ages unless they are trying to knowingly pass as younger. I would say that most Kenyans born after 1950s have and definitely after 1980 have accurate records. You have to realize that to get a national ID card, you have to produce have a birth certificate, so parents are not as reluctant as 30 years ago to make sure that they pursues a birth certificate. This is especially true for children delivered by midwives at home, but not the hospital.
You also have to realize that just like America or most countries in the world with a formal school system, these athletes are in high school or just completed high school. If you were in Kenya, you could actually see these runners when they are unknown winning high school races and that is how Kenyan teams scout for them. Even the high school that I graduated from, which is really poor in athletics, has produced two or three elite runners that won junior championships and they were discovered that way during the national high school meets in 1990s. Superior athletes in primary school are the ones who attend schools known to be good at athletics like Iten. On a side note, we used to have a very fast 10k guy. In fact, he was the fastest 10k guy anyone living then had seen in our school (not an athletics powerhouse school, moreover, our school was also in a district/province that was not very competitive). This guy ends up winning division, district and provincial championships and made it to nationals. Being very ignorant about running in general and times for 10k, the whole school was in devastated and awed when we learned that the fastest 10k guy we had ever seen in our lives was lapped once in the nationals! The only thing I can say is different from US high school system is that in Kenya, you can "repeat" a class if you fail it, so about 1-2% of students graduating may have repeated one class, making them 19 or 20 when they graduate high school. Otherwise, the system is structured so you start primary school at age 7 and finish at 14, and 14 to 18 is high school. In fact, most urban primary school were very adamant that you produce a birth certificate for admission to standard 1 (first grade) although there were exceptions and I started school in the80s!
Another thing is that even if you were born in the bushes of bushes and your parents refuse to get a birth certificate for you, there are other strong indicators of your age. One is circumcision, which for most communities used to be done at age 14. So a whole bunch of you and your age mates are circumcised together.
The points I am trying to make is that in 2013, the chances that a Kenyan doesn't know their exact birthdate, are very slim. That a Kenyan is actually older than they claim to be is not a function of "nobody knows when they were born" but more likely an attempt to age-cheat. I must say though that it is easy to get a fake birth certificate because like I said, you need it to get ID to work in Kenya, and Kenya has influx of refugees who flee camps to enter cities and therefor a huge black market exist. On the downside, Kenyans who fail to get birth certificates shortly after birth can be very frustrated because the government usually very hard on them to prove that they are Kenyans, which is next to impossible in our heterogeneous east africa where Tanzanians and Ugandans and to some Somali, Burundis, Rwandans, Congo guys speak Kiswahili.
Lastly, never judge a kenyan athlete by looking at features and claim he/she is older than they look. Believe it or not, if you took Nairobi kid and rural kid of the same age, you will notice that harshness of rural living immediately. Actually, this is a universal thing where what you have gone through, shows in your body and face.
One question I have is if they are 25 at that point winning junior championships, how comes they run for the next 10 years and still win a couple of races. Look at the current group of top Kenyan runners and go through IAAF junior competitions and you will notice their names (whether they won or not is irrelevant), more importantly, if you looked at Kenyan newspaper reports you are very likely to see when they were in high school.