Spoilers below-

Laguna is 27 years of age during his scenarios, which take place some years before the present time (assuming you're playing as Squall and company during the present time).

If you assume that Squall was conceived around the time you experience his last couple scenarios, you can add 17 to 27 and that equals out to Laguna being in his mid 40's. If you look at him during the final FMV, he clearly appears middle-aged - his skin is pale, and he has crow's feet around his eyes.