Middle-aged:
Middle age is the period of age beyond young adulthood but before the onset of old age. Though the exact range is disputed, most sources place middle adulthood between the ages of 45-65. (Wikipedia)
I think outside a professional context, the term middle-aged can be used. I know you said over 35, but that can be relative. Collins says
If you describe someone as middle-aged, you mean that they are neither young nor old. People between the ages of 40 and 60 are usually considered to be middle-aged.
Now, we could add that junior and senior are relative too. If you are 5 years older than me that makes you my senior and me your junior in certain aspects of life :-)
Consider
be 10/20, etc years sb's junior - to be 10, 20, etc years younger than someone:
That age groups are not a fixed is not surprising in our rapidly progressing world. Wikipedia says:
The United Nations defines youth as persons between the ages
of 15 and 24 with all UN statistics based on this range, the UN
states education as a source for these statistics. The UN also
recognizes that this varies without prejudice to other age groups
listed by member states such as 18–30. A useful distinction
within the UN itself can be made between teenagers (i.e. those
between the ages of 13 and 19) and young adults (those
between the ages of 20 and 29). While seeking to impose some uniformity on statistical approaches, the UN itself is aware of
contradictions between approaches in its own statutes. Hence under the 15–24 definition (introduced in 1981) children are defined as
those under the age of 14 while under the 1979 Convention on the
Rights of the Child, those under the age of 18 are regarded as
children. The UN also states they are aware that several definitions
exist for youth within UN entities such as Youth Habitat
15–32 and African Youth Charter 15–35.
On November 11, 2020, the State Duma of the Russian Federation
approved a project to raise the cap on the age of young people from
30 to 35 years (the range now extending from 14 to 35 years).