Is it good to have a boyfriend at the age of 18?

18 is well a perfect age for someone to get into relationship as at this age a person becomes enough mature to handle relationships with care and proper manner.

Is it good to be in a relationship at the age of 19?

No its pretty normal. But its also normal for 19 year olds to want to try out different boyfriends and girlfriends and partners before they settle down. Because 19 is awfully young and not everybody is ready to commit to one person at that age.