Geez... you guys aren't helping this guy!
You say you are a 1st Tenor, so I'm guessing that you are involved in a choral group or choir? Remember that the vocal cords are just a set of strings. Try (it will take time and work) to increase your range. I know that although I'm 41, over the last 2-3 years my voice has actually gotten a bit deeper... not hugely, but noticably. The only thing I did in that timeframe was to get involved in a Choir where I've had to increase my range (I'm a bass) to sing some tenor parts... the weird thing is that the more I practice the tenor parts, the deeper my voice gets.
What I'm about to say is pure conjecture, but it's the only thing I can think of. I wonder if by stretching the cords over time (to hit higher notes) has caused them to be a miniscule bit looser in normal usage, which would result in a slightly deeper tone.
So try singing on a regular basis things that are a stretch for you... both higher and lower... and see what happens.
Joe