I have mixed feelings about this. I'm 43. I'm definitely still learning things and I feel like I have some productive years ahead of me. Some of the best engineers I've ever worked with have been 45-60. That said, I feel slower now. I feel like my late 30s were a sweet spot of knowledge, wisdom and capability. I'd like to think my people skills are a bit better now though. I'm more and more suited to a lead role where I'm doing dev 30-40% and mentoring the rest of the time.