Understanding Society User Support

I work with health condition variables and try to calculate the starting age of a health condition, e.g. asthma.

The variables which I use are hcondna1 and hconda01:

hcondna1 (available in wave 10)
Label: health cond no. 1: Age told had a health condition
Age told had a health condition
Text:
What age were you when you were first told you had {if HCondncode = 6|7} a [Hcondncode]?
Universe:
if ff_ivlolw = 1 | ff_everint = 1 (interviewed at prior wave or has been interviewed previously)
and if HCONDNCODE = 1 thru 24 or 26 thru 31 or 33 thru 35 or 37 thru 42 (Has a diagnosed health condition)
What age were you when you were first told you had {if HCondncode = 6|7} a [Hcondncode]?

hconda01 (available in all waves 1-12)
Label: mc age told had health condition: asthma
Age told had a health condition
Text:
What age were you when you were first told you had {if HCondncode = 6|7} a [Hcondncode]?
Universe:
if ff_ivlolw <> 1 and ff_everint <> 1 (New entrant never interviewed)
and if HCONDCODE = 1 thru 24 or 26 thru 31 or 33 thru 35 or 37 thru 42 (Has a diagnosed health condition)

For a lot of respondents, I see identical values for hconda01 (value in the first appearance in a certain wave) and for hcondna1 (wave 10), and this is expected.

My questions:
1) What does mc in 'mc age told had health condition: asthma' for hconda01 mean?
2) I have for some respondents inconsistencies in the value of the starting age of the health condition asthma.
I would expect the same value if both variables are non-missing and if both have numeric values: for example, a respondent who is asked in all waves (w1-w12) and has for hconda01 the value 'num1' should have for hcondna1 the same value 'num1'.
For some cases, there is only a difference of 1 year in the value of hcondna1 and hconda01.
How can this be explained and which variable I should choose?
3) I see for some respondents a difference of several years for the value of hcondna1 and hconda01
How can this be explained and which value should I use for having a health condition?