How common is brain cancer or anyrisms in a 25 year old ?

Very rare: Brain cancer and aneurysms in a young person are very uncommon. A simple headache is not a cause for worry. Reasons to see a doctor are for a sudden onset, severe headache--otherwise, rest assured.

Low: Aneurysms occur in about 2% of population without risk factors. If you have one or more relatives with an aneurysm the risk can be as high as 9%. The chances of the aneurysm bleeding are about 6-16 per 100,000, ergo most aneurysms never rupture. The incidence of primary brain tumors is 28/100,000.