there are alot of changes as a dragon grows older. some are n ot as active, don't eat as much, seem to lose overal vigor, may seem stiff in the joints etc.
from what you described, it is possible he is d eveloping vision problems. he could be getting cateracts or other vision problems. it may be a good ideal to take him to a vet for an examination. the reason for his inability to catch crickets may be due to vision problems or just getting too slow to catch them in time. the vet will be able to suggest anything that could be done to improve him, or if there can be anything done. make sure the vet is very good with reptiles and bearded dragons.
as for breeding, it is a stressful time for male dragons, many forget all about eating when in pursuit of a female and mating with her. also, he may not have as effective sperm count at 5 years than he had at 2 years. though he may very well have no problems at all mating with a female or females, getting a vet check is still a good ideal, to determine overall health. then if you do wish to breed him one more time, or for the first time, you can try but watch carefully, if he he seems to be straining or having difficulties catching the female, it may be best to remove him and put him back in his old cage and look for a younger male to breed. if he does the job without showing signs of health problems, then you should be ok to continue for the season. but i don't advise breeding him again after he is 6 years old.
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PHLdyPayne