As'Salamu Alaikum

Its my pleasure to attempt answering your question , honeto.

But technicaly at what age a person is obligated to fast, and pray the five daily Salath.

This was tough for me to cite an hadith. What we generally know, is that there is no exact age to start up fastings or salah. Even as a child they can start up. If we look back at pages of seerah, we get to read that at early days of Islam,  Ali RA, would offer salah along with Prophet {sallal lahu alayhi wa sallam} and Khadija RA. Like wise we do read that Prophet {sallal lahu alaihi wasallam} would take  Anas RA, along with him to offer prayers. This shows that a child need to wait for a certain age to start up. It was reported that �Umar said: �Children�s good deeds will be recorded and their bad deeds will not be recorded.� 

Here's an other hadith from Muslim

Narrated by  Ibn �Abbaas {RA), who said: �A woman lifted up a child and said, �O Messenger of Allah, will his Hajj be counted?� He said, �Yes, and you will be rewarded.��

Scholars interpret this hadith and conclude that good deeds of a child are recorded and the bad deeds are n't accounted. Elders who help him to perform that deed are also rewarded. There is an other hadith whose reference am searching for and even you must have heard it that from seven years  wherein a child reaches an age of understanding, he shud be instructed strictly to pray, so that when he reaches his age of puberty, he shall be by then habituated to offer salah / fast.

Also if a person misses a fast for the reason of health or travel, what is the prescribed alternate compensation they must offer.

"The month of Ramadan in which was revealed the Qur'an, a guidance for mankind and clear proofs for the guidance and the criterion (between right and wrong). So whoever of you sights (the crescent on the first night of) the month (of Ramadan, i.e., is present at his home), he must observe Sawm (fasting) that month, and whoever is ill or on a journey, the same number ?of days which one did not observe Sawm (fasting) must be made up? from other days. Allah intends for you ease, and He does not want to make things difficult for you. (He wants that you) must complete the same number (of days), and that you must magnify Allah ?i.e., to say Takbir (Allahu Akbar: Allah is the Most Great)? for having guided you so that you may be grateful "

2 :185

This ayah shall give you , your answer. One who misses his fast because of  beng sick or travelling shud make it up in other days. Compensation or expiation is for those who are ill / sick and cannot make up there fast even in other days. Elderly people do fall into this category too. Read this following  ayah and hadith :

�And as for those who can fast with difficulty, (e.g. an old man), they have (a choice either to fast or) to feed a Miskeen (poor person) (for every day)� : {al-Baqarah 2:184}

The report narrated by Abu Dawood (2318) from Ibn �Abbaas concerning the verse (interpretation of the meaning):

[Ibn �Abbaas] said: This was a concession granted to old men and women who are able to fast but with difficulty, they have the option of not fasting and feeding one poor person for each day instead.

Just wanted to add on,