Page numbers are taken from the American hardback first edition...
Let us consider the time-line for Randy as follows:
We will divide Randy's life up into phases as follows:
1 - grew up and did degree
2 - hanging round working in the ILL department and playing RPGs
3 - going out with Charlene and working on the program based on Andrew Loeb's data
4 - litigation phase after that
5 - doing the masters but really hacking Unix
6 - 2nd business foray (the games company)
7 - sysadmin
8 - Epiphyte(1)
9 - Epiphyte(2)
Phases that are explicitly given times are
2 - 2 years (page 56)
3 - 18 months (page 60)
5 - 3 years (page 78)
8 - 1 1/2 years (page 187)
9 - 1 year (page 240)
But what can we deduce about the other phases?
There is nothing to justify the theory that Randy is some sort of child prodigy, in fact I would say that he is supposed to be a fairly 'average' geek. So we will assume that phase 1 lasts 21 years.
For argument's sake assume phase 4 is 6 months (I think we are being slightly generous).
6 and 7 are slightly vague in that we are told that the business consists of
A - demonstrating technology to get seed money
B - everything going well for 6 months
C - everything having gone to pot a year after that
D - Randy doing some consulting after that
but it is not completely clear at what point Randy cashes out and buys the house. One thing we DO know is that Randy buys the house for cash, therefore it can't be the case that they buy it early during 6 and then pay off a chunk of the mortgage later. It IS stated that they have lived in the house for 7 years (page 51), therefore I suppose you could try and excuse Stephenson by saying that maybe the house was a shared house he was living in, that he then bought and turfed the other lodgers out, but there is nothing to justify this.
However, for argument's sake let as assume that 6 takes 18 months and that he buys the house after 12 months - both of which imply a rather quickly developing and imploding business.
The only date I can find on a quick scan of Cryptonomicon is the date when the whois record of eruditorum.org is changed - May 1998 (page 245). This is where Randy and Enoch are having their correspondance about why they are building the crypt. It is, I think, clear that the body of the action with the Crypt, Ordo being raided etc. happens in 1999.
Note that the earthquake that happens in Randy's trip to the States where Amy persues him did not happen in our world, it is obviously a rough analog of the 1989 San Francisco earthquake but happened (I think) in 1998 in the book. I did briefly wonder whether this is supposed to be the 1994 Northridge Earthquake but, even if Randy's house is supposed to be somewhere between San Francisco and LA (and clearly the University is supposed to be somewhere south of San Francisco - Santa Cruz perhaps, which was pretty badly hit by the 89 earthquake), it is clearly closer to SF than LA. Also having the earthquake in 94 throws the chronology badly out.
What does this give us?
Summer 81 graduate and begin ILL job
Apr 83 - being program and going out with Charlene
Nov 84 - begin litigation phase
Apr 85 - start of Masters (weird time of year but there you go)
Apr 88 - begin 2nd Business Foray
Apr 89 - buy house (still in 2nd BF)
Nov 89 - begin being sysadmin (this gives us 7 years in the house, 6 1/2 of them as sysadmin)
April 96 - start of Epiphyte 1
Nov 97 - start of Epiphyte 2
Nov 98 - meeting with Sultan
This leads to some contradictions:
i) 2nd BF is supposed to have failed in a way 'typical of the early 90s-ish' (page 79) - there is some sort of 'ish' that might excuse it actually being the late 80s, I suppose
ii) he and C are supposed to have gone out for 10 years (page 28), whereas this timeline implies 13 (1 1/2 years + 6 months + 3 years + 1 year + 7 years)
iii) he is supposed to have started the program in 'the mid 80s' (page 58) - in this timeline is is mid 83 which seems a bit early to be 'the mid 80s' to me.
iv) I think (though cannot find a reference) that Randy and Avi are supposed to have had some other business dealings, when could they have fitted them in? Busy busy!
Clearly his birth date is ridiculous since he would have been conceived in 1959 when any offspring of Mary and Lawrence would have been 13 at the oldest! Even changing phase 7 to, say, 4 years still makes this implausibly young (and worth remarking on in the novel I would have said, particularly as Randy's father clearly did go to university so fathering a child when 16 would be unlikely).
It is also worth noting that Mary needs to move into a managed care facility in her early 70s which is arguably quite young for someone of her description, in the absence of any mention of any particular illness or whatever. All things considered it would make more sense if Stephenson had made Randy a late child rather than a first grandchild - I speak as someone born later than Randy of parents probably older than his grandparents.
Obviously none of this matters in the cosmic scheme of things, I mainly present it for your enjoyment. However it does seem rather cavalier for someone who has written something quite detailed in other respects e.g. the eating of breakfast cereal.