Use 'senile' carefully as it may sound disrespectful when describing elderly people. It's unkind to say he's senile just because he forgets things.
Pro Tip 2/3
Not Always Medical
'Senile' sometimes refers to age-related forgetfulness, not necessarily a diagnosed condition. Grandma's occasional forgetfulness doesn't mean she's senile.
Pro Tip 3/3
Offensive Potentials
Be cautious; 'senile' can offend if used to stereotype or mock the elderly. Calling someone senile as a joke can be hurtful.