Do I have a mature hairline or is it male pattern baldness that I should be concerned about? (Photo)

It sounds like you're dealing with a naturally high or M-shaped hairline, especially if your hairline has looked similar since your early teens. A mature hairline typically recedes slightly at the temples and then stabilizes. This is a normal part of aging and often occurs in men during their 20s. On the other hand, male pattern baldness (MPB) tends to progress over time, leading to further recession and thinning at the crown.
If your hairline hasn't changed dramatically since those early photos, it's more likely that you're experiencing the natural maturation of your hairline rather than aggressive MPB. Monitoring for any significant changes over time will help you distinguish between a maturing hairline and potential hair loss. If you notice more shedding or thinning, there are steps you can take to manage it effectively, but based on your history, this sounds more like a case of a mature hairline rather than something more concerning.