I agree with Mezo, age has nothing to do with it. What does count is that you know the bike is bullet-proof reliable and dependable.
Remember that people travel the world on things like Honda C50 stepthru's. If they can do it on these, you can do it on an older 600cc jobby.
If age and mileage does concern you why not look at getting yourself another newer XT600?
A mint Yamaha XT600E can be sourced in the UK for less than £2,000.
The venerable E model can be easily adapted for long range RTW quite easily and cheaply compared to more expensive, complicated, highly strung German options.
All you need do is fit a bash plate (£100) and an Acerbis long-range tank (£250).
For less than £2,500 you will have a legendary bike capable of almost anything.
Compare this to a German bike costing twice as much for no gain.
I recently considered outing one of my beloved XT'S for a newer XT660 but after completing a test ride on one, I quickly realised that spending £5,000 for NO gain was a stupid waste of money.
There are some other bikes which are getting long in the tooth now to consider - KLR650, XR650, AT etc... BUT the Yamaha XT600 series of bikes rule supreme when it comes to a BUDGET RTW bike (In my humble opinion).
I have owned and ridden many bikes in my life and nothing comes close to the XT legend.
I wish you well. Stay safe.
Triumph Bonneville 800 (2004), Yamaha XT600E (1999), Honda XBR500 (1986).