Glad to see you are selecting the mild end of the venomous scale. I keep mangroves (and have kept the vines).
First thing you need to do is to look at the laws in your state regarding ownership of venomous species. Make sure you can legally own one.
Second, Age does matter but not enough to prevent your keeping mildly venomous Rearfangs provided you do not have a over senitivity to venom. A good indicator is...How do you react to Bee Stings? If you have a bad reaction to those,it is a good idea not to deal with anything venomous. Also a 14 year old (depending on size) normally has a smaller body mass which would make more potent species more dangerous.
The two species you mention are not life threatening so they are good beginner snakes. Of the two I would recomend the Vine Snake first as it is easier to care for than the Mangrove and doesn't reach a size where it can envenomate you in a more serious way. A large mangrove can put some meaningful venom in you (see above).
Learning to use the tools is a very serious matter. Particularly when working with fast moving venomous snakes. What I would suggest is that you get a nice MEAN racer and practice on him. If you can move one of those around with the hooks and tongs safely, then you can upgrade to some thing more in line with your interests.
I know it sounds like a broken record but Responsibility and Patience is the key here. Working with venomous snakes obliges us to be very responsible, because what we do can affect others. So take the time to learn and progress slowly.
(end of sermon)
All that being said...Good luck with your first Rearfang!
Frank
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"The luxury of not getting involved departed with the last lifeboat Skipper..."