40 yrs young and will begin to train BJJ extensively

[bushido_man96]

I'm 46 and have been making a BJJ class pretty regularly, but only once a week right now (and will probably miss this week due to TKD testing).  Our instructor emphasizes the stand-up aspect in every class, too.  My knees are always pretty achy, but I've been holding up ok in the classes.  I'd say it's easier on my knees than TKD is.

[DarthPenguin]

I think it also depends a lot on your personal style. A lot of takedowns? likely quite hard on the knees. Similarly someone who does a lot of wrestling / scrambling and possibly uses a lot of leglocks might find it tough on the knees too. Certain guards can be tough on the knees also. Myself i am generally pretty dull style wise (usually closed guard, de la riva, bit of spider, collar / sleeve, half guard and being on top) all of which i find are not bad on the knees (though my preferences are harder on the hands tbh).

If you are worried about it being hard on the knees just try it and work out what works best for you. One of the things i like about bjj is you have that freedom to do things that work for you - if you hate armbars you just don't do them etc.!

[KarateKen]

The TKD comment makes sense, I can see that being tough on the knees as one got older.  I just found that anything grappling was tough on the knees.