Adopted a dog, not sure if the SPCA was right about his age?

I adopted a lab mix. They said Lab/Collie...now I can tell he's Lab/Husky by his curly tail, vocal talking, and how much he sheds!! When I adopted him they estimated 12-18 months. My husband and I decided we'd say he was 1 year a half at that time, so he's 2 years old now. I am seriously wondering if he may be older...say 3-5 years old. He is in good shape, good teeth, etc. Yet, he came to us with no puppy behaviours (and you'd think at 12-18 months that a lab mix would still chew and act like a puppy in some ways). He is totally mellow in the house and has never destructive chewed anything. The way that he acts you'd think he was older. So now I'm wondering if it sounds like he could be. I know there's not way to REALLY tell...but what has your experience been with adopting dogs? At 12-18 months does a lab/husky mix, in great shape, sound older than that if they do not chew anything and are low energy? Has anyone adopted a lab/husky mix before? Thanks for any thoughts. Please don't just write things like "doesn't matter, or you can never know". Personal experience and professional experience is appreciated...especially with this breed mix. Thanks.