re:Einstein is reputed to have not spoken at all until he was 5 at which time he immediately began using complete sentences. I used to find this highly doubtful, but now only mildly doubtful

while i am not going to make claims to being a genius, (but i do/did test (a rather complete, one on one test, with a psycologist) to a 165 iq in my teen years, my sister, (perhaps just as smart) did not speak till she was 3 (i was close to 5 at the time)

my mother worried she might be deaf when she didn't babble like normal babies, but the doctor did a crude test, (he gentle snapped his fingers behind G's head, and she turned to see what made the noise..)
when she did start to speak, she spoke in complete sentences. (her first words mortified my mother.. G repeated an oft heard phrase--"You god damn kids!" --my mother was upset because she thought her self moral, and she didn't realize how often she had taken the lords name in vain. (or course, its says something about my early childhood years, too, since i was one of damn kids she was always upset with!)

its not an uncommon pattern for children to not speak till they can speak in sentences. it is not a sign of genius, its just a normal pattern. some kids speak early, some late, some don't speak till they can speak in sentences.
(my kids babbled, and started to talk about 9 to 10 months, (pretty normal) but my son didn't toddle or teeter. he crawled forever--and then one day, age 15 months or so, he started to walk. day one, he walked over 2 miles non stop, no falls.. Not the usual pattern for children learning to walk, but not unknown either.

i don't think einstien was 5.. but he might well have been over the age of 3 or even 4 before he spoke, and he might well have spoken, right from the start, in perfect, or almost perfect sentences. (from a grammatical point of view) it's just how some kids behave.