Maude (age 79) is talking about mortality. The movie's plot is mostly an exploratory into life and death.
to suspend progress temporarily, as to await developments; fail to advance.
Military. to move the feet alternately as in marching, but without advancing.
With this in mind, Maude is inferring that past the age of 85, life is less significant.
"Look over the Horizon"
The phrase usually means to look beyond. With the context about mortality, this would allude to looking beyond the mortal coil.
She does prefix this by saying that age 75 is too early as her character is very much about enjoying life for what it is. This would indicate that she believes in enjoying life, but dying gracefully.
Maude maintains such a carefree outlook, in strong contrast to Harold's morbidity and is trying to convey this to Harold.
This is very much a foreshadowing of the ending:
She commits suicide by sleeping pill overdose on her 80th Birthday. The main take away is that she believes that 80 is indeed the age to die.