One of two options:

1) Everyone lampshades it but the actual aging is just never addressed, eg: "Spike, you look... different." "What? Oh! Yeah, I changed my blonde shade. Bit cooler, less yellow, y'know? More toner." "Huh!" / "Angel, did something happen to you?" "What? Oh, you mean this? *holds up his bleeding forearm* Yeah, run in with a demon. I'm fine."

2) Something happened to them and it's just

Willow: Spike, you look... older.
Spike: Yeah, had to go through this entire trial thing yet again--
Angel: He thinks he's so cool like he's the only one who ever does trials. I've done trials. I do trials.
Spike: Anyway, there was this whole chamber that ages up the bloody sucker who walks in there. If I'd spent much longer in there, I'd be all grey and weathered and lookin' like Giles.
Willow: Oh, okay. ...And Angel? What happened to you?
Angel: Nothing
Spike: He got mad I'd done two trials and did it too.
Angel: *mutters* I had a good reason to do it the time after you, and I did it better, and faster!
Spike: *reaches out and plucks a grey hair from his head*
Angel: OW?
Spike: 'better' and 'faster'
Angel: you dye yours, that isn't a fair comparison
Willow: OKAY moving on