I'm wondering about the significance of the VHS tape Leon uses the view the C-Virus footage and the 80's style computer readout. Nothing more than a simple red herring or possible indication the C-Virus is older than we think?
With regards to Jake's age, he indicates in Wesker's Report 2 that they were far too busy to consider family and he wonders how Birkin had time for one. I'd suggest Jake was born after 1991 when Wesker transferred to the Information Department and gave up his research duties.
By the time Wesker becomes STARS commander he will have walked out on his family so I'd say Jake is between 19-23 years old.