It's never too late to learn alting but at some point it's too late to learn singletapping once you're too deep into the alternating rabbit hole because your instincts will take over after various kind of 1/4 and 1/6 patterns and you will have to spend your brainpower on resisting those instincts instead of playing the map properly.
The biggest difference about it is the learning curve. Singletappers usually have a fast rise in skill as they can entirely focus on aim until they reach like 3.5* stars and then hit a massive wall in tapping hand skill and drown in despair over having to do triples and other stuff where they have to alternate.
Alternators usually progress much slower at the start as they spend a lot of time on coordinating their fingers properly with different rhythms and tapping on time but they are naturally able to perform basic 1/4 patterns by the time they reach that level of difficulty.
For that reason (made up facts follow), singletappers are generally leaning more towards maps lacking rhythmical complexity as it challenges a skill they did not take their time to develop while alternators lean more into playing just about everything because it's all the same for their fingers for the most part.
One theory I developed over time is that singletappers naturally develop a better stream speed/stamina as they have a strong habit to reset keys fast to free their main finger on 1/4 patterns, playing 2 consecutive triples like xzx xzx which is effectively a stream practice training while alternators playing xzx zxz are just lazing around on these patterns and are prone to develop less stream stamina in comparison.
Generally speaking, none keeps you from practicing rhythmically complex maps as a single tapper though, so you can always compensate. I would usually recommend people to start out singletapping unless they have a strong subjective preference for alternating as you can always switch later if you figure you need to and I think the switch is not very difficult anymore, even if you end up developing more of hybrid style as your singletapper instincts might stay.