"Dw i'n un deg pump oed."
Translation:I am fifteen years old.
General feedback, no, I don't know whether there is a standard way.
I think a comment in the language forum (e.g. Welsh from English) could be a good idea. Best if you collect various points into one post rather than one for every thing you come across.
Issues with individual sentences I'd recommend using the reporting system for. Use the freewrite option at the bottom (if you use the website version).
Note that they can't see what you entered if you use that option, so be a bit more explicit -- perhaps something along the lines of "for 'Welsh sentence' I typed 'English sentence' but it was not accepted. I think it should be because ... reasons ...."
Though for general things such as "dach should be accepted where dych is" or "why do words show up in 'choose all the correct answers' questions that we haven't been taught" or "oes should be accepted as an answer to eisiau questions" or "pymtheg should be accepted alongside un deg pump" are probably not best reported with one sentence -- those are things that affect many sentences.
Some course maintainers also welcome such general reports as messages on their timeline but I don't know whether the Welsh team does.
Yes the older (vigesimal) system is still used and is usually used with age, money and must be used with telling the time.
But I'm guessing the vigesimal system will be introduced later in the course so as not to confuse beginners who are only just getting used to the (much simpler) decimal system.
If you used "bymtheg" and it was not accepted you could report it and check "My answer should have been accepted. They would know if it should be an option. Otherwise, if you feel that more needs to be said then do as mizinamo suggests in the free write section. Most courses do accept more dialect options then they teach.
The list here does not show that bymtheg is used for fifteen but that you can say "un ar bymtheg" for sixteen and "dau ar bymtheg" for seventeen, and pedwar ar bymtheg for nineteen. For "fifteen". they show pymtheg for masculine and pymtheng for feminine. https://en.wikipedia.org/wiki/Welsh_numerals
Yes, you're right about 'hen' which is the adjective meaning old.
eg. Mae hi'n hen = She is old; Mae'r car yn hen = The car is old.
Although when 'hen' is used directly with a noun it is one of the few adjectives that precede the noun and causes soft mutation.
eg. Hen arian = Old money; but Hen gi = An old dog
Oed is a noun meaning age.
Eg 'Beth yw oed y tŷ?' = 'What is the age of the house?' - This tends to be transcibed into 'How old is the house?
When we are saying someone is 'un deg pump oed' we're saying literally 'fifteen of age' which is transcribed into English as 'fifteen years old'