Suddenly thinning at almost 40

[KR77]

Hi, I am a 39 year old who has had thick hair all my life. I never really worried about hair loss, mostly because my hairline was and still in pretty intact. However I noticed that all of a sudden my crown and top of my head seem to be significantly thinning.
Can rapid hair loss occur at my age??
I would say I'm almost a stage 3 vertex but still with a straight hairline. Should I be concerned about going bald quickly as opposed to gradually over many years?