Mi abuelo tiene setenta años, pero sigue subiendo las escaleras sin problema.

Because in Spanish, age is expressed with tener (to have), not ser (to be).

• Mi abuelo tiene setenta años = My grandfather is seventy years old
• Literally: My grandfather has seventy years

This is one of the first big differences English speakers notice:

• Tengo veinte años = I am twenty
• ¿Cuántos años tienes? = How old are you?

So ser setenta años would be wrong here.

You usually say setenta años when giving someone’s age.

• Tiene setenta años = standard and complete
• Tiene setenta may be understood in context, but on its own it sounds incomplete or less natural

Spanish normally includes años after the number when talking about age.

Sigue subiendo means he keeps climbing or he still goes up.

This is the structure:

• seguir + gerund

It expresses that an action continues or is still happening.

Examples:

• Sigue trabajando = He/She is still working
• Sigo estudiando español = I’m still studying Spanish
• Seguimos esperando = We’re still waiting

So in your sentence, the idea is that even at seventy, the grandfather still continues to climb the stairs.

Because after seguir, Spanish normally uses the gerund to show an ongoing or continuing action.

• seguir + gerund = to keep / continue doing something

So:

• sigue subiendo = he keeps climbing
• not sigue subir

The form subiendo is the gerund of subir.

Compare:

• Estoy subiendo = I am going up / climbing
• Sigue subiendo = He keeps going up / climbing

Sigue comes from the verb seguir (to follow, to continue).

It is irregular in the present tense because the e in the stem changes to i in many forms:

• yo sigo
• tú sigues
• él/ella sigue
• nosotros seguimos
• vosotros seguís
• ellos siguen

So sigue is the he/she/it form here, matching mi abuelo.

Because Spanish often leaves out subject pronouns when they are already clear from the verb or context.

• Mi abuelo tiene setenta años, pero sigue...
• You do not need él sigue... because mi abuelo is already the subject

Adding él would usually only be for emphasis, contrast, or clarity.

This is very normal in Spanish:

• Tengo hambre = I’m hungry
• Vivimos en Madrid = We live in Madrid

Spanish often uses the definite article where English may not.

So:

• subir las escaleras = to go up the stairs / to climb stairs

In Spanish, las escaleras is the normal way to say it. English is more flexible here.

Also, escaleras is usually plural because a staircase is thought of as a set of steps:

• Subir las escaleras
• Bajar las escaleras

It most directly means to climb/go up the stairs.

Depending on context, English could translate it as:

• to climb the stairs
• to go up the stairs
• sometimes simply to go upstairs

But the Spanish specifically mentions the stairs, so the image is of physically using the stairs rather than, for example, taking a lift.

Sin problema is a very common fixed expression meaning without difficulty, no problem, or easily.

In Spain especially, sin problema sounds very natural.

You may also hear:

• sin problemas
• sin ningún problema

All are correct, but they feel slightly different:

• sin problema = very common, compact, idiomatic
• sin problemas = without any difficulties/problems
• sin ningún problema = without any problem at all, a bit more emphatic

So in this sentence, sin problema means he can still do it with no trouble.

Because pero means but, and it links two contrasting ideas:

• Mi abuelo tiene setenta años
• pero sigue subiendo las escaleras sin problema

The comma helps separate those two parts, just as in English you would often write:

• My grandfather is seventy years old, but he still climbs the stairs easily.

This is standard punctuation.

Pero introduces a contrast.

The contrast is:

• he is seventy years old
• but he still climbs the stairs without difficulty

It suggests that the second idea is noteworthy given the first. In other words, the speaker is implying that many people at that age might find it harder.

Yes, you could, and it would be very natural:

• todavía sube las escaleras = he still goes up the stairs
• sigue subiendo las escaleras = he keeps / continues going up the stairs

They are similar, but not identical:

• todavía focuses on still
• seguir + gerund focuses on continuing the action

In this sentence, sigue subiendo gives a strong sense of continued ability and activity.

In this sentence, yes: mi abuelo clearly means my grandfather.

The word abuelo can sometimes also refer more generally to an elderly man in certain contexts, but when you say mi abuelo, it normally means my grandfather.

Related words:

• abuelo = grandfather
• abuela = grandmother
• abuelos = grandparents

Yes: it contains the letter ñ, which is a different letter from n in Spanish.

• año = year
• ano = anus

So the tilde matters a lot here.

Plural:

• año = year
• años = years

In age expressions, you almost always use it in the plural unless the number is one:

• tiene un año
• tiene setenta años