It is possible to get diagnosed as dementia at a young age also though it is not so common. Many medical conditions that cause dementia in older age may sometimes happen at young age also rarely. Dementia is not just forgetting things or events or names  , but may occur as difficulty performing work or tasks, behavioral changes , speaking or writing language issues , changes in way of interaction etc. But keep in mind that any dementia especially at young age ,may be due to treatable conditions like vitamin deficiency (vitamin  B12 ) thyroid or hormone issues, infections , some treatable brain diseases like wilsons , Toxins like alcohol use etc. Again some other conditions may seem to be dementia , for example depression. So first consult a Neurologist with detailed history and evaluation and do tests to rule out these conditions.