The key word here is "ben mehagrim" status. This status is obtained by Israeli citizens who fulfill the following conditions:
A “ben mehagrim” is a person
whom the Army classifies as child of Israeli citizens residing
overseas who departed Israel with their parents before the age
In order to maintain this status one must fulfill the following conditions:
Ben Mehager must be in Israel no more than 120 days in a calendar year
There's also the option of spending up to 365 days in Israel without losing the status:
A Ben Mehager is granted a one-time Shnat Shehiya (a year in Israel which will not affect one’s Age of Arrival). This is defined as a visit to Israel for a duration of 121-365 days. Prior to using a Shnat Shehiya a Ben Mehager must be outside of Israel for 60 days, and they must stay out of Israel for the 60 days following their year. This is in order to create a clear distinction between visits to Israel.
Nefesh B'Nefesh then provides a convenient chart detailing the length of time in the army, depending on the age:
So to answer the questions:
- They can visit for up to 120 days per calendar year, or visit for up to 365 days at once without breaks.
- They're exempt from the age of 28, unless they're doctors or dentists.