Feeling lonely in my 40s

I'm feeling so lonely in my 40s. I've been able to hold onto my best friend from high school and college for a while, but we've grown apart this last year (I'm actually serving as his best man in a few weeks...). I've also been single for the 3 years and able to keep busy with work and my 3 kids. I feel like I can't connect with people anymore and it's really concerning to me. I can't seem to make friends as I've able to do before. Everyone seems to be into completely different things than me these days and I'm at a loss... Am I just being a grumpy old man? Any advice?