There is a difference between how this is calculated in the ILR vs. how it is calculated in the OData field.
The AgeAtStart field in the ILR is calculated as the learner's age on the 31st August of the learning period funding year.
The OData field is calculated as the learner's age on the Programme Start date.
The full name of OData field is PersonalInformation_AgeAtStart and the data is taken from the Programme Details information. The system takes Episode Start Date only, which can be set up on the Add/Edit Programme Details page only. As far as the OData field is concerned, note that the system does not take into consideration the ILR Main Aim's Learning start date.