Can a man age 47 still get a women pregnant

Me and my live in girlfriend live together she recently found a new job (past three months) back in febuary the first week we had sex one time . the next time was the last week we went to get test done (positive) 11 weeks as of the 16 of april if my calculations are right then this means I am not the boilogical father right or wrong during the middle of the month we had problems we would argue and her stories would not ad up she would go to work early and work late also she would fix herself up real nice well mid march she said that she had a problem a male coworker lately she has been reall tense and nervous and she is always daydreaming is it possible she had an affair we have been together 2 years with no birth control