Totally. The only real reason 'Robin' is associated with being a kid is because there's been a parade of children filling Dick's boots ever since he became Nightwing. Had he always been Robin, I think 'Robin' would mean something much different. Robin is Dick's identity, and nothing about it demands that it be a child's identity. It's supposed to be in the vein of Robin Hood, a swashbuckler- which Dick surely is.
Batman is free to have other associates but they don't need to be kids and they don't all need to be called Robin. Hell, Dick being a grown up Robin out on his own would lend extra gravitas anytime he teams up with Batman again- BATMAN AND ROBIN, the band is back together.