time and money are big factors. the pup will need tons of one on one time for socialization (on his own outings with the human) and to some extent individual training. The older dog may have lots of work needed as well (then management when they are together/training together). if you have time for that, no harm in going for it!
The other factor is you will in turn (should all go well) have two seniors - who may not be able to work much (and then not at all) so if you love the sport and being active, you will need a third dog or take a break (which may suit you fine). Personally I like having a dog i their "prime" at all times and can't have more than two dogs. so for me it means more of an age gap.
Also sometimes senior dogs are much more expensive. totally depends on the dog though. But Rocky is costing me over $100/mt in pain management and stuff. So two seniors could mean that x2.